add support for double-tap to go back a transition

by replacing the onClickListener with an onTouchListener with the simple
variant of the GestureDetector.

Also drop the check that prevents going to the "end-of-presentation,
click to exit" slide and exiting the presentation that way.

Change-Id: I54b49bf11929ad9415b8c85581fe16998ab3a7a7
Reviewed-on: https://gerrit.libreoffice.org/7107
Reviewed-by: Andrzej Hunt <andrzej.hunt@collabora.com>
Tested-by: Andrzej Hunt <andrzej.hunt@collabora.com>