extensionUtils: Add DISABLING and ENABLING extension states

Extensions can export asynchronous enable() and disable()
functions. To guard against re-entrancy when enabling or
disabling an extension, this commit adds two new states:
ENABLING and DISABLING which are set immediately prior
to calling enable() and disable() respectively.

This commit updates the extensions CLI and Extensions app
with new strings for these states.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2364>

data/dbus-interfaces/org.gnome.Shell.Extensions.xml

@@ -77,6 +77,8 @@
                 <member>4: OUT_OF_DATE</member>
                 <member>5: DOWNLOADING</member>
                 <member>6: INITIALIZED</member>
+                <member>7: DISABLING</member>
+                <member>8: ENABLING</member>
                 <member>99: UNINSTALLED</member>
               </simplelist>
             </para></listitem>