ALT-<KEY> commands for VI-style navigation (ALT-<H/J/K/L>)

[leighleighleigh]

This PR adds ALT-key-combo commands, as an alternative to the default CTRL-G command-mode shortcut.
This was done mostly out of personal preference - they require less keystrokes to activate (and more ergonomic, imo).

So far I've added the following 'alternate' commands:

• ALT-<K/J/H/L>, change focus UP/DOWN/LEFT/RIGHT between windows
• ALT-O, move focus to the previously active window

Additional handling for ESCAPE keys was also added. This is because ESC and ALT are hard to distinguish.
These changes improve the responsiveness of pressing ESC, e.g, while using vim.

I'm not super familiar with this code base, but it doesn't seem to have broken anything in my (albeit light) usage :)