Doc: Add snippet for DragHandler.acceptedButtonsHEADdev

DragHandler docs have been showing TapHandler code snippets, because
both DragHandler and TapHandler inherit properties from
PointerDeviceHandler, which is abstract and uses TapHandler code
snippets in some of its property documentation.

Override the acceptedButtons property documentation by adding it to the
DragHandler class, and add a testable snippet.

Task-number: QTBUG-119866
Pick-to: 6.8 6.7 6.5 6.2
Change-Id: Ic2c8ee98c33929fd90c8ec11509d510647c4a074
Reviewed-by: Andreas Eliasson <andreas.eliasson@qt.io>

0 files changed, 0 insertions, 0 deletions