Improve sidebars

[ShirleyNekoDev]

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)

[leMaik]

Probably related to #1274?