(search) Keep menu open when selecting tags

(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:
{F7853717}

- `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