server/shadow: Build libraries for shadow subsystems

server/shadow: Build libraries for shadow subsystems

Commit 60ae27b0 decoupled subsystem implementations and shadow framework
core by moving subsystem specific code right into freerdp-shadow-cli.
This, however, doesn't allow applications that would like to link
libfreerdp-shadow, in order to embed RDP shadow server, to reuse also
the subsystem module.

3rd party developers now have to either provide their own subsystem code
(copied from FreeRDP sources or written from scratch) or be limited to
usaging freerdp-shadow-cli executable, which doesn't expose all
functions of the shadow server library (e.g. enumeration of available
monitors).

This change moves the shadow subsystem out of the executable into new
freerdp-shadow-subsystem library, which freerdp-shadow-cli and
potentially other applications can link to.