Presentation: optionally go to prev. page when touching left half of the page

This patch makes switching presentation slides using touch screen taps
more flexible.  It introduces a new configuration option
'Touch navigation' with three possible values:
* 'Tap left/right side to go back/forward':
   Tapping on the left(right) half of the screen
  makes the presentation go to the previous(next) slide.
* 'Tap anywhere to go forward':
  Tapping on the screen makes the presentation go to the
  next slide, no matter where the screen is being tapped
* 'Disabled':
  Screen tapping doesn't do anything.

Previously, only 'Tap anywhere...' was implemented.

This patch does not change the behavior of mouse clicks.

Differential Revision: https://phabricator.kde.org/D18118