Add more granular groups in keybindings menu

Currently we split all keybinding menus (accessed via the '?' key) into local, global, and navigation groups:

I want a more granular grouping scheme, _something_ like what I have on the experimental `more-section-headers` branch:

Applying this across the board is pretty simple, but deciding on the design is hard. For example:
* in what order should we show groups? I think navigation, misc and custom commands should go last (not shown in the above image)
* Do we include local and global keybindings in the same group? If so, should we order it so that local comes before global within a group. Should we style them differently?
* How granular do we go?
* What groups should we use? The screenshot shows diff display options in the same group as the diff menu which is arguably a completely different thing.