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: {F7305382}

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: ngraham, kfm-devel

Tags: #dolphin

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