Replace custom override schema with per-desktop override

GSettings now recognizes per-desktop overrides that can be used
to change schemas' default values for a particular desktop. This
is not entirely unlike our existing custom override mechanism in
mutter, except that it is not limited to keys in org.gnome.mutter,
and it doesn't require a separate schema - the latter means that
we (and gnome-teak-tool) no longer have to figure out the correct
schema for the current login session and just use the original one.

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

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

@@ -0,0 +1,6 @@
+[org.gnome.mutter:GNOME]
+attach-modal-dialogs=true
+edge-tiling=true
+dynamic-workspaces=true
+workspaces-only-on-primary=true
+focus-change-on-pointer-rest=true

data/meson.build

@@ -92,6 +92,7 @@ schema = configure_file(
   configuration: schemaconf,
   install_dir: schemadir
 )
+install_data('00_org.gnome.shell.gschema.override', install_dir: schemadir)
 
 if have_systemd
   unitconf = configuration_data()

data/org.gnome.shell.gschema.xml.in

@@ -190,6 +190,7 @@
     </key>
   </schema>
 
+  <!-- unused, change 00_org.gnome.shell.gschema.override instead --!>
   <schema id="org.gnome.shell.overrides" path="/org/gnome/shell/overrides/"
 	  gettext-domain="@GETTEXT_PACKAGE@">
     <key name="attach-modal-dialogs" type="b">