Don't activate workspace when more than one window is open

Don't activate workspace when more than one window is open

Currently clicking on an empty spot on the workspace actors switches to the workspace, this has a side effect that when for some reason (like using a touchpad),
the user misses the window he intends to open he will end up with the currently focused window instead.

Disable this behaviour if more than one window is open (in that case the user has to explicitly target a specific window).

https://bugzilla.gnome.org/show_bug.cgi?id=610868