Fixes a noisy reposition of SC.PickerPane.

The pane listened to changes to its borderFrame in order to know whether to reposition itself or not. This means that changes to its size (width/height) allowed it to reposition to best fit, but because repositioning ends up changing the same borderFrame, it meant one (potentially dangerous) extra call through positionPane for no reason. Instead, we use viewDidResize, which only gets called on size (width/height) changes.
Fixes several inconsistencies in property referencing within SC.PickerPane. There were multiple places where '.' notation was used instead of get/set. Also tidies up some unclear code.