feat: add tray icon menu options for switching the icon

Description

As of 1b71d2cb (v6) we install three tray icon variants ulauncher-indicator-symbolic, ulauncher-indicator-symbolic-dark and ulauncher-indicator, but only the first one is used.

It's possible to change the icon manually by setting status_icon_name in ~/.config/ulauncher/settings.json to one of these icon names or any other installed icon, but it would be nice to be able to switch between them in the context menu for the tray icon.

My Gnome extension that adds tray icons also makes them monochrome and adds a black background instead of the transparent one, so it doesn't make much sense to switch from the default, so I'm leaving this task for others. It should be quite simple (see AppIndicator.py).

Communication guidelines

[X] I have read and agree with Ulauncher's communication guidelines and checked the existing feature requests before suggesting this.

Feature isn't possible to support as an extension.

[X] This feature request cannot satisfactory be supported as a third party extension.

Ulauncher v6

[X] I have checked and couldn't find my suggestion in v6.

Ulauncher / Ulauncher