Ubuntu Touch/Lomiri support

[Semphriss]

Ubuntu Touch being GNU/Linux, it already mostly supports SDL, but many APIs are not well supported, mostly modern system- and DBus-related stuff.

Would it be worth to add greater support for Lomiri? After a quick look into it, I could add support for:

• Light/Dark theme (already done, actually)
• Clipboard (Full, with MIME)
• System tray
• Camera
• Screen saver
• URL Dispatching (SDL_OpenURL)

...as well as some not-yet-available features:

• Notifications (Add System Notification Support #1932, Add SDL notifications for linux using DBus portal.Notification #7857)
• App icon bubble count and progress bar ([Feature Request] Add update progress bar ? #7901)
• Sharing between apps (Emscripten port feature request: Share action #5368?)
• Geolocalisation (SDL Location API #7849)

[slouken]

Sure, it seems like implementing existing features would be a good idea. The not-yet-available features can wait. :)