[Dolphin] Open Preferred Search Tool action

Summary:
Added "Open Preferred Search Tool" action to Tools menu.

It runs preferred (topmost) external search tool as configured in the "More Search Tools" menu.

By default Ctrl+Shift+F shortcut is assigned to this action.

FEATURE: 384798
FIXED-IN: 20.03.80

Reviewers: Dolphin, ngraham, elvisangelaccio

Reviewed By: Dolphin, ngraham

Subscribers: pkloc, kfm-devel, kde-doc-english

Tags: Dolphin, Documentation

Differential Revision: https://phabricator.kde.org/D22594