[Layer Shell Protocol] Add support for "exclusive-edge" feature

[marcusbritanicus]

Version 5 of wlr-layer-shell-unstable-v1 adds a new feature: set_exclusive_edge. This helps the compositor to resolve the edge to which exclusive zone is applied when the layer surface is anchored to the corner. From the xml file:

Requests an edge for the exclusive zone to apply. The exclusive
edge will be automatically deduced from anchor points when possible,
but when the surface is anchored to a corner, it will be necessary
to set it explicitly to disambiguate, as it is not possible to deduce
which one of the two corner edges should be used.

The edge must be one the surface is anchored to, otherwise the
invalid_exclusive_edge protocol error will be raised.