extensionSystem: Allow disabling session mode extensions

Trying to disable an extension that is enabled by the session mode
currently has no effect, which is clearly confusing. We could update
the various extension UIs to reflect that via sensitivity, but being
unable to configure extensions based on which session the user picked
at login isn't obvious either.

So instead, add a 'disabled-extensions' gsettings key to list extensions
that should not be enabled which takes precedence over 'enabled-extensions'
and can be used to disable session mode extensions.

https://bugzilla.gnome.org/show_bug.cgi?id=789852

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

@@ -21,6 +21,17 @@
         EnableExtension and DisableExtension D-Bus methods on org.gnome.Shell.
       </description>
     </key>
+    <key name="disabled-extensions" type="as">
+      <default>[]</default>
+      <summary>UUIDs of extensions to force disabling</summary>
+      <description>
+        GNOME Shell extensions have a UUID property; this key lists extensions
+        which should be disabled, even if loaded as part of the current mode.
+        You can also manipulate this list with the EnableExtension and
+        DisableExtension D-Bus methods on org.gnome.Shell.
+        This key takes precedence over the “enabled-extensions” setting.
+      </description>
+    </key>
     <key name="disable-user-extensions" type="b">
       <default>false</default>
       <summary>Disable user extensions</summary>