entry: Use gestures to handle pointer/touch events

entry: Use gestures to handle pointer/touch events

Similarly to GtkTextView, a GtkGestureMultiPress gesture handles
button/touch presses to initiate one selection mode or other, and
a GtkGestureDrag is used to handle text selection and DnD checks.

The code from button press/release, motion, and grab_notify handlers
has been shuffled into the actions triggered by those gestures.