Add set_cursor_style keybind action

[maedmatt]

With the new key tables and vim-mode keybindings (#3708), there's no visual indicator when a key table is active.

A set_cursor_style action would allow cursor changes as mode indicators:

keybind = alt+v=activate_key_table:vim,set_cursor_style:block
keybind = vim/escape=deactivate_key_table,set_cursor_style:bar

This would complement the planned vim mode improvements mentioned in #3708.