Currently we only relayout when the screen size changes, this gets
the cases where a monitor gets added/removed but not when the primary
monitor changes.
We need to relayout on all monitor layout changes.
Remove ShellGlobal::screen-size-changed signal as it is no longer used, Gdk is
used to track changes now.
A ShellGlobal::gdk-screen property is added for this purpose.
https://bugzilla.gnome.org/show_bug.cgi?id=620377
ccbf247970