Add ScrollLinkMap for diff scrolling in folder comparisons (bgo#729756)

In file comparisons, scrolling in the LinkMap flips between changes,
but does nothing in folder comparisons. While it's a bit weird to
have this behaviour on an otherwise blank widget, it doesn't hurt
anything and does feel somewhat more consistent.

Author: kaiw

data/ui/dirdiff.ui

@@ -257,11 +257,11 @@
               </packing>
             </child>
             <child>
-              <object class="GtkDrawingArea" id="linkmap1">
+              <object class="ScrollLinkMap" id="linkmap1">
                 <property name="width_request">50</property>
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
-                <property name="events">GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK</property>
+                <property name="events">GDK_SCROLL_MASK</property>
                 <property name="vexpand">True</property>
               </object>
               <packing>
@@ -315,11 +315,11 @@
               </packing>
             </child>
             <child>
-              <object class="GtkDrawingArea" id="linkmap0">
+              <object class="ScrollLinkMap" id="linkmap0">
                 <property name="width_request">50</property>
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
-                <property name="events">GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK</property>
+                <property name="events">GDK_SCROLL_MASK</property>
                 <property name="vexpand">True</property>
               </object>
               <packing>

meld/dirdiff.py

@@ -1398,6 +1398,9 @@ def set_num_panes(self, n):
                 idx = 1 if i else 0
                 w.setup(scroll, self.get_state_traversal(idx), [self.fill_colors, self.line_colors])
 
+            for w in self.linkmap:
+                w.associate(self)
+
             for widget in (
                     self.vbox[:n] + self.file_toolbar[:n] + self.diffmap[:n] +
                     self.linkmap[:n - 1] + self.dummy_toolbar_linkmap[:n - 1]):

meld/linkmap.py

@@ -122,3 +122,20 @@ def do_draw(self, context):
 
     def do_scroll_event(self, event):
         self.filediff.next_diff(event.direction)
+
+
+class ScrollLinkMap(Gtk.DrawingArea):
+
+    __gtype_name__ = "ScrollLinkMap"
+
+    def __init__(self):
+        self._setup = False
+
+    def associate(self, melddoc):
+        self.melddoc = melddoc
+        self._setup = True
+
+    def do_scroll_event(self, event):
+        if not self._setup:
+            return
+        self.melddoc.next_diff(event.direction)