preferences: New pref for applying text filters in folder comparison

New preferences are awful, but this has been a long-standing issue for
performance of folder comparisons, and it's often not clear to new
users that it's a thing we do. Also, the upcoming newline-ignoring
change makes this even more expensive, so now seems like the right time
to add this.

Author: kaiw

data/org.gnome.meld.gschema.xml

@@ -161,6 +161,11 @@
           <summary>File timestamp resolution</summary>
           <description>When comparing based on mtime, this is the minimum difference in nanoseconds between two files before they're considered to have different mtimes. This is useful when comparing files between filesystems with different timestamp resolution.</description>
       </key>
+      <key name="folder-filter-text" type="b">
+          <default>true</default>
+          <summary>Apply text filters during folder comparisons</summary>
+          <description>If true, folder comparisons that compare file contents also apply active text filters and the blank line trimming option, and ignore newline differences.</description>
+      </key>
       <key name="folder-status-filters" type="as">
           <default>['normal', 'modified', 'new']</default>
           <summary>File status filters</summary>

data/ui/preferences.ui

@@ -787,6 +787,22 @@
                                 <property name="position">1</property>
                               </packing>
                             </child>
+                            <child>
+                              <object class="GtkCheckButton" id="checkbutton_folder_filter_text">
+                                <property name="label" translatable="yes">Apply text filters during folder comparisons</property>
+                                <property name="visible">True</property>
+                                <property name="can_focus">True</property>
+                                <property name="receives_default">False</property>
+                                <property name="use_underline">True</property>
+                                <property name="xalign">0</property>
+                                <property name="draw_indicator">True</property>
+                              </object>
+                              <packing>
+                                <property name="expand">False</property>
+                                <property name="fill">False</property>
+                                <property name="position">2</property>
+                              </packing>
+                            </child>
                           </object>
                           <packing>
                             <property name="expand">False</property>

meld/preferences.py

@@ -199,6 +199,7 @@ def __init__(self, parent):
             ('use-system-editor', self.system_editor_checkbutton, 'active'),
             ('custom-editor-command', self.custom_edit_command_entry, 'text'),
             ('folder-shallow-comparison', self.checkbutton_shallow_compare, 'active'),
+            ('folder-filter-text', self.checkbutton_folder_filter_text, 'active'),
             ('folder-ignore-symlinks', self.checkbutton_ignore_symlinks, 'active'),
             ('vc-show-commit-margin', self.checkbutton_show_commit_margin, 'active'),
             ('vc-commit-margin', self.spinbutton_commit_margin, 'value'),
@@ -212,12 +213,15 @@ def __init__(self, parent):
         for key, obj, attribute in bindings:
             settings.bind(key, obj, attribute, Gio.SettingsBindFlags.DEFAULT)
 
-        settings.bind(
-            'use-system-editor', self.custom_edit_command_entry, 'sensitive',
-            Gio.SettingsBindFlags.DEFAULT | Gio.SettingsBindFlags.INVERT_BOOLEAN)
-        settings.bind(
-            'use-system-font', self.fontpicker, 'sensitive',
-            Gio.SettingsBindFlags.DEFAULT | Gio.SettingsBindFlags.INVERT_BOOLEAN)
+        invert_bindings = [
+            ('use-system-editor', self.custom_edit_command_entry, 'sensitive'),
+            ('use-system-font', self.fontpicker, 'sensitive'),
+            ('folder-shallow-comparison', self.checkbutton_folder_filter_text, 'sensitive'),
+        ]
+        for key, obj, attribute in invert_bindings:
+            settings.bind(
+                key, obj, attribute, Gio.SettingsBindFlags.DEFAULT |
+                Gio.SettingsBindFlags.INVERT_BOOLEAN)
 
         self.checkbutton_wrap_text.bind_property(
             'active', self.checkbutton_wrap_word, 'sensitive',