Implement native SDL tray

[C0rn3j]

Will eventually fix two issues - #1316 and #1904, after SDL gets a better tray implementation - see libsdl-org/SDL#14582 about that.

Some reading material is also in libsdl-org/SDL#11703 and libsdl-org/SDL#13666

Commits with the ayatana glib implementation were removed, in case that's actually desired later for some odd reason - 1772.patch

Currently it seems to be blocked by AyatanaIndicators/libayatana-appindicator-glib#87

We could still improve on this by using SDL instead.

• Get show/hide working - waiting on SDL response to see if this will be possible
• Get accessiblity strings working - waiting on SDL response to see if this will be possible
• Get Windows/macOS icons sorted out - do we even have a tray on macOS? On Windows we'll need to tear out lynxtray
• Get scroll up/down to change volume - waiting on SDL implementation
• Implement sdl_tray_set_label / sdl_tray_set_title