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.
Use the offset you just wrote in...!
Merge back from develop