Add shortcut commands for underline & sub/superscript.

Underline is particularly important as CE will apply underline
formatting automatically when you press Ctrl+U but the
SurfaceObserver will not notice it, leading to inconsistency
between the view and the model.

For sub/superscript I've used the Google Docs key mappings as these
appear to have the fewest conflicts with existing browser shortcuts
and there isn't much consistency between desktop clients anyway
(Word and Open/LibreOffice use completely different shortcuts).

Bonus: reordered command lists to be consistent with UI layout.
Change-Id: I92998e42f9bcfb932d44e8f483811efd538e5981
Merge "Add shortcut commands for underline & sub/superscript."