Toggle focus-follows-mouse by configurable keystroke

[TreyBlancher]

Until the age-old auto-raise feature is overcome, or keeping floating windows always on top of tiled windows gets implemented, it'd be really nice if we could configure an arbitrary key chord (including modifiers) to toggle focus-follows-mouse. That way we could turn it off to access the floating window that keeps disappearing, then turn it back on, all without having to go to the Amethyst preferences and unchecking/rechecking the box.

This is especially bad with some SalesForce windows. At least for me, clicking on some search fields brings up a popup window. Without focus-follows-mouse, this new popup gains focus (and Amethyst usually makes it tile). But with focus-follows-mouse, the original SalesForce window retains focus, and the popup destroys itself detecting it no longer has focus. Geniuses at SalesForce, let me tell ya. Amethyst is slow enough that I'm forced to disable tiling while I access that popup.

Trello Card

[ianyh]

This was implemented in #453, but was not added to preferences.