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
Fixing build.
Merge pull request #2670 from SickheadGames/Multiselect

PipelineTool treeview multiselection support.