Improve sidebars

Current sidebar is initially sized depending on the window size without proper automatic resizing of the inner control elements. This introduces multiple problems:
- if too narrow: cutting off overflowing controls (and not generating horizontal scrollbars to inform the user about it)
- if too wide: much unused whitespace
- also not the nicest design if every element has varying width

Suggested solution:
- make all controls auto-resizable to stretch to the full width
- make sidebars collapsible
- give sidebar (or it's content) a minimum width
- (retain feature to resize sidebar (up to a reasonable width))
- (add scrollbars to content if sidebar gets resized below minimum width)
- save and restore absolute sidebar sizes on Chunky restarts (not relative to window width)