Terminal plugin: add default shortcut F4 for Toggle Focus

Summary:
(Toggle Focus switches the focus between the editor component
and the Terminal Tool View.)
F4 was chosen because it is consistent with Dolphin (where F4
opens and closes the Terminal Panel) and there is no conflict with
other shortcuts.

Reviewers: Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, sars, kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D16652