data: Use override for default alt-tab keybindings

Per-desktop overrides aren't limited to keys in org.gnome.mutter, so
we can use them instead of the alternate-tab extension to default to
the window switcher in the classic session.

https://bugzilla.gnome.org/show_bug.cgi?id=786496

data/00_org.gnome.shell.extensions.classic.gschema.override

@@ -3,3 +3,7 @@ dynamic-workspaces=false
 
 [org.gnome.desktop.wm.preferences:GNOME-Classic]
 button-layout='appmenu:minimize,maximize,close'
+
+[org.gnome.desktop.wm.keybindings:GNOME-Classic]
+switch-applications=[]
+switch-windows=['<Super>Tab','<Alt>Tab']

meson.build

@@ -34,7 +34,6 @@ endif
 uuid_suffix = '@gnome-shell-extensions.gcampax.github.com'
 
 classic_extensions = [
-  'alternate-tab',
   'apps-menu',
   'places-menu',
   'launch-new-instance',
@@ -43,6 +42,7 @@ classic_extensions = [
 
 default_extensions = classic_extensions
 default_extensions += [
+  'alternate-tab',
   'drive-menu',
   'screenshot-window-sizer',
   'windowsNavigator',