First version of updated key map editor UI (in User Preferences)

First version of updated key map editor UI (in User Preferences)

Now the key maps are displayed in a hierarchical list which you can
browse through. As well as in the main list, modal key maps are also
available in context, for example, if you unfold out a Transform key
map item, you'll be able to fold out and access its modal key map underneath.

More work to do, including search, better operator browsing, etc.
Still need to revise the ordering/hierarchy and clean up naming to be
consistent too, it's a bit of an 'evolved' mess right now.

Thanks to theeth for some initial work here too.