Allow configuring click behavior in Desktop Grid effect

The following click behaviors are defined:
 * Switch desktop and activate window [default]
 * Switch desktop only

The former emulates the previous default of activating clicked windows
using the Present Windows effect. The latter introduces a new mode where
the clicked window is not activated, allowing the user to select a
virtual desktop without worrying about disrupting the last active
window.

The configuration toggle that controlled the use of the Present Windows
effect has been subsumed into the first click behavior, i.e., if the
effect is enabled, it will be implicitly triggered.