Let the terminal panel dock to any part of the window

Summary:
BUG: 362593

Just what it says: Let the terminal panel sit on the left or right side of the window.

Test Plan:
Tested in KDE Neon:

• Opened terminal panel, unlocked widgets, and tried to drag it to the right or left side of the window: I can
• Put terminal panel on right side, quit Dolphin and opened Dolphin again: terminal panel appears on the right as expected
• Put terminal panel on right side, closed terminal panel, quit Dolphin, opened Dolphin, and opened the terminal panel again: terminal panel appears on the right as expected
• Removed dolphinrc file and opened terminal panel: appears on the bottom

Reviewers: Dolphin

Subscribers: elvisangelaccio, anthonyfieroni, cfeck

Differential Revision: https://phabricator.kde.org/D10189