Fix setup of search shortcut

Summary:
The setup of the search shortcut was seriously broken.
Since multiple NavigationActionBars were instantiated
for each view, the shortcut would also be instantiated
multiple times, which could even crash Elisa.

Test Plan:
toggling the search fieed works, Elisa does not crash
when using the shortcut with different view

Reviewers: ngraham, ognarb, mgallien

Reviewed By: ngraham, ognarb

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D22781