Shadow Server

Start consolidating screen sharing servers (xfreerdp-server, wfreerdp-server, mfreerdp-server) as a single "shadow" server.

Shadow server:
* Create core framework with subsystem architecture
* Add better encoding queue management
* Add support for automatic generation of TLS certificate.
* Add support for Standard RDP security
* Add better multiple monitor management

Shadow (X11):
* Migrate xfreerdp-server to shadow subsystem
* Add support for invalid region detection when xdamage is unusable

Shadow (Windows)
* Migrate Win8+ DXGI 1.2 support (unstable, disabled)
* Add support for Windows Desktop Sharing API (Vista+)
* Add WDS + RDP client wrapper, allowing easy frame buffer access

FreeRDP core:
* Add support for WinPR synchronized barriers
* Fix client and server-side unix domain socket support as TCP alternative
* Fix and improve remote assistance invitation parsing
* Add basic encomsp server-side support