Add Dolphin integration
Summary: Add Dolphin integration.
All unit tests passed. Special icons and context menu entries are correctly shown and working. Plus, I've been using it for three weeks.
nextcloud-client alone works well without the extension.
Reviewers: Triage Team, JoshStrobl
Reviewed By: Triage Team, JoshStrobl
Subscribers: DataDrake, JoshStrobl
Differential Revision: https://dev.getsol.us/D4779