Allow dolphin to auto-play previewed media file, click on preview to play/pause video...

Allow dolphin to auto-play previewed media file, click on preview to play/pause videos or audio

Summary:
It is based on D19844.

I did my best to avoid glitches hence the amount of code touched.

Retry after @pekkah D7539
Moved the setting to the information panel context menu, no more timer

Settings screenshot :
{F6700220}

This would mach the same feature in the open/save dialog (although not equivalent)
{F6696456}

FEATURE: 378613
FIXED-IN: 19.08.0
GUI: New information panel context menu option

Test Plan:
Without auto play
  - in dolphin with the information panel opened, and the auto media play feature is disabled (right on the information panel)
  - hover over media files
  - the behavior is the same as before the patch

With auto play
  - in dolphin with the information panel opened, and the auto media play feature is enabled
  - hover over media files
  - media is played automatically
  - hover over another media file, the new media is previewed

Use audio or video file as media.

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, elvisangelaccio, ngraham

Subscribers: ngraham, broulik, kfm-devel, pekkah

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19782