remove .rect member (clashes with QWidget)

FigureCanvasQTAgg inherits from QWidget, which already has a 'rect'
property and getter.

Also, consolidate .rect and (boolean) .drawRect into a single _drawRect
member.  This is only used for the zoom rect and not accessed from other
classes (other than indirectly via the drawRectangle() function).
update api_changes.rst w.r.t. Qt4 backend changes
explicitly test _drawRect against None
Merge pull request #2946 from hmeine/fix_rect_clash

remove .rect member (clashes with QWidget)