Merged requested to merge work/dont_collapse_toolviews into master
bccc7d35 hides plugin views when their sidebar is collapsed which made the sidebar unresizable.
This commit adds a placeholder (an empty
QLabel) which is shown
when the sidebar is collapsed so that it will still be resizable.
The plugin views are properly shown again after the sidebar is
manually expanded using the mouse.
This is the fix I mentioned in !451 (comment 270707).