shell: Fix view repositioning logic for output move and destroy

shell: Fix view repositioning logic for output move and destroy

Previously, the repositioning logic would iterate the compositor's list
of layers and move the views on those layers. However, that failed in
two different ways: it didn't cover hidden workspaces and crashed when
the display was locked.

This patch changes the logic to explicit iterate over all the layers
owned by the shell. The iteration is done through a helper function,
shell_for_each_layer().

https://bugs.freedesktop.org/show_bug.cgi?id=76859
https://bugs.freedesktop.org/show_bug.cgi?id=77290