Interim draft of QmitkMultiLabelInspector adding modify operations
and old version of context menu.
Missing:

• user based renaming
• goto label support
• layer specific context menu
• implemented support for multiselection mode and modification locks (for use as pure selection widget in e.g. segmentation utils)

Other stuff:

• added more fine grained ItemModelRoles to QmitkMultiLabelTreeModel
• added fine grained modification support to QmitkMultiLabelTreeModel
• added additional index methods to QmitkMultiLabelTreeModel