filediff: Use symbolic icons for the locked-file button

Author: kaiw

data/ui/filediff.ui

@@ -327,7 +327,7 @@
                     <property name="can_focus">False</property>
                     <property name="receives_default">False</property>
                     <property name="tooltip_text" translatable="yes">This file can not be written to. You may click here to unlock this file and make changes anyway, but these changes must be saved to a new file.</property>
-                    <property name="icon_name">emblem-readonly</property>
+                    <property name="icon_name">changes-prevent-symbolic</property>
                     <signal name="toggled" handler="on_readonly_button_toggled" swapped="no"/>
                   </object>
                 </child>

meld/filediff.py

@@ -1569,6 +1569,9 @@ def set_buffer_editable(self, buf, editable):
         buf.data.editable = editable
         index = self.textbuffer.index(buf)
         self.readonlytoggle[index].set_active(not editable)
+        self.readonlytoggle[index].props.icon_name = (
+            'changes-allow-symbolic' if editable else
+            'changes-prevent-symbolic')
         self.textview[index].set_editable(editable)
         self.on_cursor_position_changed(buf, None, True)
         for linkmap in self.linkmap: