Add tooltips to krunner configure and close buttons

Summary:
Shows tooltips for buttons left and right of the search box.

FEATURE: 398943

Test Plan: Manually test appearance of tooltips, unlikely to cause new issues.

Reviewers: VDG, acrouthamel, ngraham

Reviewed By: VDG, acrouthamel, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15749