Update visual parts for tags.

Summary:
If you have categories, we need to find the corresponding tags and
afterwards, we need to show this the user.

There is no need to call onSelectionChanged, because this oly would
trigger the incidence as dirty. This is not true, because the user
hasen't changed anything, if we load the categories or if a new tag is
created in background.

Reviewers: mollekopf

Maniphest Tasks: T1276

Differential Revision: https://git.kolab.org/D175