- add selectByRect( QgsRectangle&, SelectBehaviour) and
selectByIds( QgsFeatureIds, SelectBehaviour) for selecting
by rect and ids respectively, with options to add to selection/
remove from selection/intersect with current selection
- deprecate select( QgsRectangle ) and setSelectedFeatures in
favour of new methods
- add unit tests
- use at() instead of [] for temporary containers (at() returns
const reference, so it avoids the detach which occurs with [])
- use at( 0 ) rather than first() for containers, because first()
detaches (Qt5 introduced constFirst()/constLast() to overcome this)
