Added multiple item selection support to the treelist control
Minor code cleanups.
Treelist multiselect is now only valid for siblings.  Collapsing selections selects the parent is now restored.  Fix for theme-less flicker on mouse move.
Eliminated extraneous iteration steps.  Fixed Shift key handling.  Selected count now internally tracked rather than calculated.
Renamed multiselect style macro.  Ensure internal selection data is updated on item delete.
NULL checks on selected item in key handling routines.
Merge pull request #10 from ArmstrongJ/treelist-mutliselect

[TREELIST] Merging support for item multi-selection.