appDisplay: Open new window on middle-click

The current middle-click action of opening a new window on a new
workspace is a bit peculiar; it is also not overly hard to achieve
the same result by moving a new window to the desired workspace or
selecting a workspace before opening a new window. Just opening
a new window is also a more common action, so having a shortcut
available that doesn't require a modifier is a good idea as well;
change the middle-click behavior accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=695010