Add Open Terminal Here action from dolphin to menu

Depends on frameworks/kio!210 (merged)

We can also utilize this for reorganizing the entries in the context menu: https://phabricator.kde.org/T13752

A complaint about that bug can be found here: https://www.reddit.com/r/kde/comments/iefoip/how_to_change_default_terminal_for_dolphin/

By using the existing dolphin action we also give the user a nice hint on how to do the same thing using a shortcut:

@elvisangelaccio