Reenable Label tooling

[JustusBraun]

This PR adresses #42 and does the following things:

• GPU accelerated selection of Faces using an offscreen Ogre render pass (Removes the need for OpenCL)
• 3 different selection modes:
  • Single click: (De)select a single face under the cursor
  • CTRL + Click: (De)select faces using a circular brush
    • Holding CTRL and using the Scrollwheel changes the size of the brush
  • SHIFT + Click: (De)select faces using a selection rectangle
• Labeled instances are written to the /mesh/labels/LABEL_NAME/INSTANCE_NUMBER dataset
• LabelTool and LabelVisuals respect the Culling Mode selected on the MeshDisplay
• Existing Instances can be edit and overwritten

[amock]

Nice additions!