[dolphin] Add an action to toggle the searchbar

Summary:
Make search action toggle the searchbar instead of just launching it.

The search action in dolphin did only bring up the search bar, but to close it again you had to go to the closing button on the same searchbar.
This behavior in inconsistent with other dolphin actions which toggle panels or tools.

BEFORE:
{F7256652}

AFTER:
{F7256862}

BUG: 344617
FIXED-IN: 19.12.0
Closes T8473

Depends on D23075

Test Plan:
- Enable the search mode: the searchbar appears and the toolbar button gets checked
- Press the search button again, and it goes back to normal mode.
- The search button state is coherent with the searchbox
- Coherence is kept when changing to a split view or different tab
- Shorcut <Ctrl-F> does not close the searchbar, but moves the focus to it.

Reviewers: #dolphin, ngraham, #vdg, elvisangelaccio

Reviewed By: ngraham, #vdg

Subscribers: ndavis, felixernst, elvisangelaccio, kfm-devel

Tags: #dolphin

Maniphest Tasks: T8473

Differential Revision: https://phabricator.kde.org/D23232