appDisplay: Don't grow indicators more than the parent

Currently the indicators are a BoxLayout inside a BinLayout in AllView.
BinLayout doesn't have any size constraint, so if the indicators request
a bigger size than AllView the entire overview is grown, causing the
overview to go crazy.

To avoid that, create an actor for the page indicators that request as
minimum size 0, and as a natural size, the sum of all indicators natural
sizes. Then we clip_to_allocation, so it doesn't grow more than the
parent.

https://bugzilla.gnome.org/show_bug.cgi?id=723496