Add action for focusing Terminal Panel

Summary:
Add an action for focusing and de-focusing the Terminal Panel.

FEATURE: 185096
FIXED-IN 20.04.0

Test Plan:
- Hit {key Ctrl Shift F4} or click {nav Tools > Focus Terminal Panel} or {nav Control > Tools > Focus Terminal Panel}
- If the Terminal Panel was closed, it opens and gains focus
- If the Terminal Panel was open but unfocused, it gains focus
- If the Terminal Panel was open and focused, focus returns to the view

{F6630289, size=full}

Reviewers: #dolphin, elvisangelaccio, rominf

Reviewed By: #dolphin, elvisangelaccio, rominf

Subscribers: kfm-devel, elvisangelaccio, rkflx, ngraham, #dolphin

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D10959