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.
Fixed missing space before /launchdebugger command.
Fixed Build/Rebuild menu items to not be drop downs.
Unified tree view to use a single context menu for both folders and items.
Fixed item rebuild to work on multiple selection.
Added "Debug Mode" checkbox to enable content pipeline debugging.
Made multiselect properties non-browsable by the designer.
Removed setting of selected nodes.
This hides the selection properties from the designer serialization process.
Fixed up deletion of items to remove empty folders.
We can no longer remove the root item.
Merge branch 'pipeline7' into NewContent

Conflicts:
	Tools/Pipeline/Common/IController.cs
	Tools/Pipeline/Pipeline.csproj
	Tools/Pipeline/Windows/MainView.Designer.cs