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)