Enhancement: Redesigned account and folder management view

The account and folder management panel was rewritten with a new
model/delegate architecture. It features improved error display, better
tree navigation, context menus for folder actions, and visual polish
throughout.

https://github.com/owncloud/client/pull/12399
https://github.com/owncloud/client/pull/12407
https://github.com/owncloud/client/pull/12441
https://github.com/owncloud/client/pull/12456
https://github.com/owncloud/client/pull/12463
https://github.com/owncloud/client/pull/12468
https://github.com/owncloud/client/pull/12472
https://github.com/owncloud/client/pull/12477
https://github.com/owncloud/client/pull/12478
https://github.com/owncloud/client/pull/12480
