[Expo] Add zoom and toogle grid on/off, and more

Among other things, this patch does the following:

* Add quick zoom to the currently selected workspace thumbnail, toggled with the z key.
* Add gradual zoom:
    * Ctrl+'+' (plus)/Ctrl+mouse-wheel up: zoom in
    * Ctrl+'-' (minus)/Ctrl+mouse-wheel down: zoom out
    * Ctrl+'0' (zero): zoom off
* Add toggle of grid view on/off, activated with the g key

I don't intend to push this a lot further on my own. I would appreciate help with making these features more easily discoverable. The animations when zooming could also need a helping hand.