Add composited transparency

[onli]

Simdock should use real transparency for its background when a compositor is active.

This would fix #9 and likely #6.

Before now, this was impossible since the wxwidgets compositing feature was broken. Maybe it's fixed now, after the move to GTK3 in #11. However, an initial test did not work -> enabling transparency for the frame background in wxWidgets still seems to be completely broken. But maybe that's not true, and someone knows how to get it to work. Comments, PRs etc welcome.

[onli]

And just like that - it works! The solution was what I linked in #9 (comment): SetBackgroundStyle(wxBG_STYLE_TRANSPARENT); before the custom wxFrame gets created.

I will check that the composited approach actually works in IceWM (it not working in other docks was the main factor in me maintaining this project), if not it will get a setting. Update: No issues in icewm.