Move gestures overview from qtdoc to qtbase; mention Qt Quick
On the one hand this doc reads like an overview, but didn't mention
Qt Quick; on the other, the gestures framework is questionable,
and in any case is solidly in the widgets module, not reusable for
Qt Quick. So, just added some comments at the end to make it clear
that Qt Quick takes a different approach. Also changed the relevant
links because the title has changed.
Change-Id: I66a0c0c106f496de26fb8947e90826ef39ccfddd
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>