meldwindow: Stop using configure-event

According to https://wiki.gnome.org/HowDoI/SaveWindowState, we should
be using size-allocate and window-state-event.

Author: kaiw

data/ui/meldapp.ui

@@ -7,7 +7,8 @@
     <property name="title" translatable="yes">Meld</property>
     <property name="icon_name">meld</property>
     <signal name="delete-event" handler="on_delete_event" swapped="no"/>
-    <signal name="configure-event" handler="on_configure_event" swapped="no"/>
+    <signal name="size-allocate" handler="on_size_allocate" swapped="no"/>
+    <signal name="window-state-event" handler="on_window_state_event" swapped="no"/>
     <child>
       <object class="GtkVBox" id="appvbox">
         <property name="visible">True</property>

meld/meldwindow.py

@@ -410,15 +410,22 @@ def on_next_diff_changed(self, doc, have_prev, have_next):
         self.actiongroup.get_action("PrevChange").set_sensitive(have_prev)
         self.actiongroup.get_action("NextChange").set_sensitive(have_next)
 
-    def on_configure_event(self, window, event):
-        state = event.window.get_state()
+    def on_size_allocate(self, window, allocation):
+        gdk_window = window.get_window()
+        if not gdk_window:
+            return
         nosave = Gdk.WindowState.FULLSCREEN | Gdk.WindowState.MAXIMIZED
-        if not (state & nosave):
+        if not (gdk_window.get_state() & nosave):
             width, height = self.widget.get_size()
             variant = GLib.Variant('(ii)', (width, height))
             settings.set_value('window-size', variant)
 
+    def on_window_state_event(self, window, event):
+        state = event.window.get_state()
+
+        # TODO: Handle fullscreen
         maximised = state & Gdk.WindowState.MAXIMIZED
+        # fullscreen = state & Gdk.WindowState.FULLSCREEN
         window_state = 'maximized' if maximised else 'normal'
         settings.set_string('window-state', window_state)