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