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 Ctrl+⇧+F4 or click Tools → Focus Terminal Panel or 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

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