Style

* background of MaskablePixmapWidget now depends on the average color of the pixmap (the goal is not to please everyone's flavor!)
* issue #334 a)
* extended Style to allow symbols like @mediumGrey -> #c1c1c1
* removed some cases where QPalette was used for styling (styling should only be done in .css files, some platform-styles don't even use palettes)
* some changes to the chatlog
* updated .css files (more to go)

Hopefully this doesn't cause any regressions :smile: