Add possibility to define unified background areas

[NyaomiDEV]

I love the fact that this widget provides me with an easy way to have "floating islands", however I'd very much like to unify the background between the system tray and the clock.

The way I envision it is by adding a "background unifier" widget that unifies parts of the panel's custom backgrounds; so by adding this "special separator" between the systray and the clock, the background island between the two is shared.

(Or probably the inverse can be done, using panel separators also as background separators)

[luisbocanegra]

I like this idea, I think someone in the kreddit post suggested this but I forgot to make an issue for it.

I have to think of the implementation yet but a way I imagine it is something like this:

Widget unifier/Join widget backgrounds

1. System tray Clock
2. Window Buttons Window Title
3. Select start widget Select end widget
4. Add new

[luisbocanegra]

Actually, I think it makes more sense to implement it as the special separator you suggested, so it will work like the margins separator widget does.

1. Iterate through the widgets
2. If the "background unifier" is found start merging backgrounds
3. If the "background unifier" is found stop merging them