[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

{F7134238}
{F7134240}
{F7134242}

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