Performance improvement FlatScrollBar

Setting of FlatScrollBar configuration values without state changes (e.g. fsb.setThumb(fsb.getThumb()) triggered an unnecessary layout.
Yourkit analysis showed that this had a huge performance impact on
scrollableAdapter implementation.

Now only state changes lead to layout triggering.
Adjust FlatScrollBar selection on thumb changes
Merge commit 'a3fc6fe0b7cfbcad7c7dbd9199250a77c88678f8' into development