Added support for $(symbols) in the MGCB

This will allow the user to use

 bin\$(Config)\$(Platform)

for OutputDir and IntermediateDir
Correcting response file syntax to require "/@:" as intended.
Merge pull request #3034 from dellis1972/MGCBSymbols

Added support for $(symbols) in the MGCB
Fixed issue in MGBuildParser.ShowUsage for properties.
Merge pull request #3035 from SickheadGames/PLTResponseFileUsage

Correcting response file syntax to require "/@:" as intended.
work in progress for the new plugin for xamarin studio
Various updates
Fixed Build Error
Moved stuff around
Updated to latest
Gtk Pipeline Changes