Adding option to open externally called folder in a new tab

Summary:
Adds an option to open externally called folder in a new tab.

By default this option is enabled

Test Plan:
If option selected:
1. All valid arguments passed to Dolphin should be opened in tabs of an instance(s) (if it exists). Duplicate tabs just change activation to current tab.

If option not selected:
1. All calls to Dolphin result in a new instance being opened

This option does not require Dolphin to be restarted to take effect.

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: broulik, ngraham, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D21736