filediff: Remove cycle-pane action in favour of prev/next (bgo#701665)

Cycling panes has always felt a little bit weird, and really it's very
straightforward to use a previous/next model instead.

Author: kaiw

data/ui/filediff-ui.xml

@@ -51,7 +51,8 @@
         <menuitem action="LockScrolling"/>
       </placeholder>
       <placeholder name="ViewDocSpecificPlaceholder">
-        <menuitem action="CycleDocuments" />
+        <menuitem action="PrevPane" />
+        <menuitem action="NextPane" />
       </placeholder>
     </menu>
   </menubar>

data/ui/filediff.ui

@@ -155,12 +155,20 @@
       </object>
     </child>
     <child>
-      <object class="GtkAction" id="CycleDocuments">
-        <property name="label" translatable="yes">Cycle Through Documents</property>
+      <object class="GtkAction" id="PrevPane">
+        <property name="label" translatable="yes">Previous Pane</property>
+        <property name="tooltip" translatable="yes">Move keyboard focus to the previous document in this comparison</property>
+        <signal name="activate" handler="action_prev_pane" swapped="no"/>
+      </object>
+      <accelerator key="Page_Up" modifiers="GDK_MOD1_MASK"/>
+    </child>
+    <child>
+      <object class="GtkAction" id="NextPane">
+        <property name="label" translatable="yes">Next Pane</property>
         <property name="tooltip" translatable="yes">Move keyboard focus to the next document in this comparison</property>
-        <signal name="activate" handler="action_cycle_documents" swapped="no"/>
+        <signal name="activate" handler="action_next_pane" swapped="no"/>
       </object>
-      <accelerator key="Escape" modifiers="GDK_CONTROL_MASK"/>
+      <accelerator key="Page_Down" modifiers="GDK_MOD1_MASK"/>
     </child>
     <child>
       <object class="GtkToggleAction" id="LockScrolling">

meld/filediff.py

@@ -503,6 +503,11 @@ def on_current_diff_changed(self, widget, *args):
         self.actiongroup.get_action("CopyLeftDown").set_sensitive(copy_left)
         self.actiongroup.get_action("CopyRightUp").set_sensitive(copy_right)
         self.actiongroup.get_action("CopyRightDown").set_sensitive(copy_right)
+
+        prev_pane = pane > 0
+        next_pane = pane < self.num_panes - 1
+        self.actiongroup.get_action("PrevPane").set_sensitive(prev_pane)
+        self.actiongroup.get_action("NextPane").set_sensitive(next_pane)
         # FIXME: don't queue_draw() on everything... just on what changed
         self.queue_draw()
 
@@ -734,11 +739,8 @@ def _corresponding_chunk_line(self, chunk, line, pane, new_pane):
 
         return new_line
 
-    def action_cycle_documents(self, widget):
-        pane = self._get_focused_pane()
-        new_pane = (pane + 1) % self.num_panes
+    def move_cursor_pane(self, pane, new_pane):
         chunk, line = self.cursor.chunk, self.cursor.line
-
         new_line = self._corresponding_chunk_line(chunk, line, pane, new_pane)
 
         new_buf = self.textbuffer[new_pane]
@@ -747,6 +749,16 @@ def action_cycle_documents(self, widget):
         self.textview[new_pane].scroll_to_mark(
             new_buf.get_insert(), 0.1, True, 0.5, 0.5)
 
+    def action_prev_pane(self, *args):
+        pane = self._get_focused_pane()
+        new_pane = (pane - 1) % self.num_panes
+        self.move_cursor_pane(pane, new_pane)
+
+    def action_next_pane(self, *args):
+        pane = self._get_focused_pane()
+        new_pane = (pane + 1) % self.num_panes
+        self.move_cursor_pane(pane, new_pane)
+
     def _set_external_action_sensitivity(self):
         have_file = self.focus_pane is not None
         try:
@@ -1862,7 +1874,6 @@ def set_num_panes(self, n):
                 widget.hide()
 
             self.actiongroup.get_action("MakePatch").set_sensitive(n > 1)
-            self.actiongroup.get_action("CycleDocuments").set_sensitive(n > 1)
 
             def coords_iter(i):
                 buf_index = 2 if i == 1 and self.num_panes == 3 else i