Vertically center tooltip icon/image

Summary:
Right now the widget tooltip's icon/image is top-aligned, which in practice makes it
vertically centered when the tooltip has only one line of text, but not vertically
centered when there are two lines of text.

This patch makes it vertally centered all the time, improving the appearance.

Test Plan:
Before:

After:

Reviewers: VDG, Plasma, filipf, ndavis

Reviewed By: VDG, filipf, ndavis

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27299