Created a mouse wheel scroll method for scrollbars.  They work similarly to button scrolls, however in the event the user constructs a scrollbar without buttons it won't cause a crash like the previous implementation.
Removed the previous mouse wheel handling logic for Scrollbars.
Moved the RenderLayerType enum to the UI namespace, from the RenderLayer class.
Updated the UIManager to signal to the focus when a mouse drag has started and ended.
Documentation fix.
Removed the mouse dragging events and related code from the Widget, also renamed the drag check method so it doesn't refer to the widget itself being dragged.  The base widget class only needs to worry if the mouse is dragged, it is then up to the implementation to determine what that means for the specific widget.
Silly compile fixes.
Added creation methods for both the TabSet and EditBox to the Screen class.
Completed the initial version of the EditBox widget.
Merge branch 'master' of github.com:BlackToppStudios/Mezzanine