Replace the stop button of the media preview by a pause button

Summary:
Stop is not as useful as paused plus it makes the button coherent with the click to play/pause feature.

CCBUG: 364957

Test Plan:

Reviewers: Dolphin, elvisangelaccio, ngraham

Reviewed By: Dolphin, ngraham

Subscribers: ngraham, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D23648