Improve thumbnail tooltip animation

Summary:
This fixes a cosmetic issue where the tooltip got 'stuck' as long as you
kept moving your mouse around. The trade-off is it more often than not
fades out quicker than it did before, but I believe this isn't a problem.

This prevents the tooltip fade-out animation getting the 500ms delay unless the current animation is not running.
Therefore preventing the animation perpetually gaining a delay if moving the mouse over thumbnails fast enough.

BUG: 245091

Before:

After:

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D10704