Add a Shadow protocol

The shadow protocol is inspired by the KWin's X11 protocol and the
DecorationShadow in KDecoration2.

A shadow is attached to a surface (after a commit on surface) and
consists of several image parts (represented through a buffer):
* top-left
* top
* top-right
* right
* bottom-right
* bottom
* bottom-left
* left

In addition there is an offset on each side.

For more information see also the X11 protocol described at [1].

Note: the protocol is currently missing documentation and changing
the shadow is not yet properly delegated to the server API.

[1] https://community.kde.org/KWin/Shadow