Split out several pieces of code into independent functions that could
be used in other chart classes. Added several convenience methods for
combining multiple selections and dealing with mouse modifier keys
combined with chart level options for selection mode.

Some slight modification to the modifier key logic, with shift giving
addition, control giving subtraction and both keys pressed giving the
toggle behavior. No key pressed results in a new selection.

Change-Id: I4fe80987cbd2937b587b44bd094a9adea439eb24