Change: Removal of selective sync tree from account status page

We removed the selective sync support from the main widget and moved it to its own dialog,
available through the context menu of the sync folder.

Having both the sync status and the selective sync settings in the same widget caused a lot of problems and overhead over the years.
Additionally, it required us to maintain a lot of special and basically duplicated code, as we already had a dedicated widget to handle selective sync.

https://github.com/owncloud/client/issues/11357
https://github.com/owncloud/client/issues/11357
https://github.com/owncloud/client/issues/8102
https://github.com/owncloud/client/issues/5423
https://github.com/owncloud/client/issues/4080
https://github.com/owncloud/client/pull/11524
