Added preprocess step for response files.

$set Name=Value
$if Name=Value
$endif
Update mgcb.md

Documented /set, $if, and $endif
Merge branch 'develop' into PipelinePreprocess

Conflicts:
	Documentation/mgcb.md
Can now multiselect within the treeview.
Delete action will affect all selected items, including folders.
RebuildItem changed to RebuildItems, works on all selected treeview items (folder selection means all items within it are built).
Merge branch 'develop' into Multiselect

Conflicts:
	Tools/Pipeline/Common/PipelineController.cs
	Tools/Pipeline/Windows/MainView.Designer.cs
Merge pull request #2669 from SickheadGames/PipelinePreprocess

Added preprocess step for response files.
Fixing build.
Merge pull request #2670 from SickheadGames/Multiselect

PipelineTool treeview multiselection support.