focus should follow alt-tab

[Lachlan Andrew]

I use focus-follows-mouse. When I "alt-tab to" a different window, I expect the focus to go to that window, regardless of the position of the mouse. Currently, it raises that window, but focus stays wherever the mouse is.

Rationale:
A: Switching focus is the familar behaviour from Gnome and Windows
B: Switching focus is the most useful behaviour. The natural reason to use a keyboard shortcut is to use the keyboard without wanting to touch the mouse; what point is there in seeing a window without being able to interact with it?