display: avoid unnecessary stack changes

Currently docks are raised when a mouse enters the window and are
lowered when mouse leaves it. Typically this will make unnecessary
stack changes and unneeded screen redraw.

Functions meta_window_raise and meta_window_lower raises or lowers
windows within the window layer. For dock windows that means that
raising/lowering happens between windows in META_LAYER_BOTTOM layer
or META_LAYER_DOCK/META_LAYER_TOP layers.

In typical configuration with top and bottom panels this means that
rasing/lowering happens between both panels for no reason. Stop doing
that if dock does not overlap with other windows in same layer.

Dock raising was added in commit 7be4c63e when panel was put in
the nromal layer.