Split the style sheet out from Widget.py.

Having a huge amount of literal text in amongst the code was getting quite unwieldy.
Reimplemented MultiSelectionMenu using MenuButton.

They both need to do the same thing, and there's no point in duplicating the code. I do wonder if MultiSelectionMenu shouldn't _have a_ MenuButton rather than _be a_ MenuButton, but since it derived from Button previously I've kept it this way for backwards compatibility (ChannelMaskPlugValueWidget for one is relying on this).
Made MenuButtons raise menus on press rather than release.

Fixes #742.
Added menu indicator to MenuButton.

Fixes #493.