gapplication: add a way to override the app-id

Some applications support running in a mode where they present
themselves as a different application to the user (for example web
browsers or terminals).

To facilitate this, add an option --gapplication-app-id which allows
users to override an application's id from desktop files or similar.

Applications need to opt-in to this by setting the
G_APPLICATION_CAN_OVERRIDE_APP_ID flag.

https://bugzilla.gnome.org/show_bug.cgi?id=743933

gio/gioenums.h

@@ -1464,6 +1464,9 @@ typedef enum
  *     owner of the application ID nor does it check if an existing
  *     owner already exists.  Everything occurs in the local process.
  *     Since: 2.30.
+ * @G_APPLICATION_CAN_OVERRIDE_APP_ID: Allow users to override the
+ *     application ID from the command line with `--gapplication-app-id`.
+ *     Since: 2.48
  *
  * Flags used to define the behaviour of a #GApplication.
  *
@@ -1479,7 +1482,9 @@ typedef enum
   G_APPLICATION_HANDLES_COMMAND_LINE = (1 << 3),
   G_APPLICATION_SEND_ENVIRONMENT    =  (1 << 4),
 
-  G_APPLICATION_NON_UNIQUE =           (1 << 5)
+  G_APPLICATION_NON_UNIQUE =           (1 << 5),
+
+  G_APPLICATION_CAN_OVERRIDE_APP_ID =  (1 << 6)
 } GApplicationFlags;
 
 /**