Fix - panes adjustment slider
Fix - panes adjustment slider

The panes slider for adjustment left and right panes were broken after
adding flex box changes. The changes in this commit fixes this and adds
a number of improvements (no more fixed height for the bar, no buggy
behaviour when dragged too far, bar is flush with the parent).

arkon/Markus,MarkUsProject/Markus,MarkUsProject/Markus,benjaminvialle/Markus,MarkUsProject/Markus,benjaminvialle/Markus,benjaminvialle/Markus,MarkUsProject/Markus,arkon/Markus,benjaminvialle/Markus,MarkUsProject/Markus,benjaminvialle/Markus,MarkUsProject/Markus,arkon/Markus,arkon/Markus,arkon/Markus,arkon/Markus,benjaminvialle/Markus,benjaminvialle/Markus,arkon/Markus,arkon/Markus,MarkUsProject/Markus,MarkUsProject/Markus,arkon/Markus