Minor doc fixes.
Another batch of fixes and convenience additions.  Newly created RenderLayers can now easily be added to the Normal and Hovered RenderLayerGroups by specifying the respective ZOrders on construction.
Completed the initial version of the GridContainer widget.
Made a few touchups to the catch UI configuration.  Also added the GridContainer to one of the places it needs to be.
Merge branch 'master' of github.com:BlackToppStudios/Mezzanine