Rework master loop

Rework master loop
Implement thread safe MessageQueue
All postMessage* calls replaced with a single safe-to-call anywhere postMessage()
Rewrite main command loop to use new locking around MessageQueue
Deleted Command::masterMutex
Use condition variable in MessageQueue/Command to wait on rather than going into Remote
Implemented new thread loop in Remote. Reports input events by Message
Completely replaced signal handling system
Revert Linux compiler to GCC
Revert Linux linker to GCC LD

This will be broken for Windows - remote class needs updating