Make share target consistent when grouping group share with user share

In some situations, a group share is created before a user share, and
the recipient renamed the received share before the latter is created.
In this situation, the "file_target" was already modified and the second
created share must align to the already renamed share.

To achieve this, the MountProvider now groups only by "item_source"
value and sorts by share time. This makes it so that the least recent
share is selected as super-share and its "file_target" value is then
adjusted in all grouped shares.

This fixes the issue where this situation would have different
"file_target" values resulting in two shared folders appearing instead
of one.