VIM-98 Invoke actions in separate commands for better Undo/Redo

Vim actions used to be invoked inside a key typing command and didn't
have their own commands. Now they've got separate commands with unique
group IDs in order to stop them from being merged into combined
Undo/Redo steps.

The Undo/Redo behaviour still differs from Vim: typing in a single
Insert mode session is split into several Undo chunks depending on
timing. An Undo step may require one more Undo step for moving the
cursor to its position before Undo.