appDisplay: Only allow ctrl-click for apps that can open new windows

Trust the heuristics in shell_app_can_open_new_window() to get it right
more often than not, and add an appropriate check in activate(). This
makes the behavior consistent with the dash, e.g. we will try to open
a new window (and show the corresponding animation) for apps that don't
have a "New window" item in their dash context menu.

https://bugzilla.gnome.org/show_bug.cgi?id=736329