Tray Icon revamp

[oniGino]

Hey, I'd like to submit a PR for changing how you handle tray icons
currently the "monocrome" or black tray icon looks not so great on my desktop.

I think it should be replaced with something like this
https://github.com/syncthing/syncthing-android/blob/master/graphics/ic_stat_notify_full.svg

That way no matter what color your "dark" theme is, the tray will look good.
We could even invert it, then that could be the "light colored theme"
then you could change the option from "monocrome" to "Dark theme" or something like that..

What do you think about this PR? I'd like your input before I start on it.