Allow client to specify desktop screen size

[Antoine Martin]

• Add default screen size option to Settings
  
• Add combo to SessionStartDialog? when used with full desktop commands
  
• Add screen-size parameter to START_SESSION protocol
  
• Handle screen size requirements in VNC and NX:
  
  • NX can just specify it with the nx/nx options
    
  • Xvnc: either specify it on command line or use xrandr to change resolution of pre-loaded sessions.