Added new CALayer category helper method -mm_boundingRect
#Refactoring in MMScrollBarLayer

- removed dependency from a CAScrollLayer
- knob positioning and sizing is handled via scrollBarDelegate
#Refactoring in MMScrollBarLayer, MMFlowView

- click offset in knob considered when dragging
- added item clicking support
#Refactoring in MMScrollBarLayer

- cleaned up mouse dragging
Drag image support

- removed dead code