(search) Keep menu open when selecting tags

Summary:
Keeps the Tags menu open while selecting and deselecting tags in the
search panel, saving many mouse clicks for multiple tags.
It cannot be done natively on QMenu (https://bugreports.qt.io/browse/QTBUG-6635)
but it is a one-liner and I haven't seen any bad behavior so far.

Test Plan:

• Tags menu does not close when selecting/deselecting tags
• Tag search is consistent
• Clicking anywhere else will close the menu

Reviewers: Dolphin, elvisangelaccio, ngraham, VDG

Reviewed By: Dolphin, elvisangelaccio, VDG

Subscribers: VDG, broulik, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D26343