This makes the `Toggle{Left,Right,Bottom}Dock` actions deserializable
from empty JSON, so that they can be constructed for the command
palette. It also fixes a bug in GPUI's `available_actions` method, in
which we'd include key bindings for actions of the same type but
different values.

Note that, for now, the command palette will perform the *focusing*
version of the actions. I'm not totally sure this is the right behavior,
but it seems more useful to me.

Release Notes:

N/A