Allow non-interactive focus cycling.

Adds an <interactive>bool</interactive> option to the NextWindow and
PreviousWindow actions. When it is false, the action is not interactive and will
immediately switch focus to whatever the next focus target is.

Removing the "interactive" flag from the focus_cycle() method, as it was unused
previously, and the new code does not make use of it either. In order to be
non-interactive it simply starts a focus_cycle then immediately ends it when the
action ends.

The "interactive" flag in focus_cycle() forced a linear cycling order which may
not be what you want, so the new method is preferrable anyhow.