Note: This also removes "has:" filters, since they are not applied
on database query level.

JIDs can be long, and have characters which are not allowed in the
filesystem. This splits the jid on domain / localpart / resource and
generates a folder structure. Further each part is limited in length.

- Create file per jid
- Fix FileChooserButton not correctly returning the selected directory in some circumstances

- Create the rows only once and use _filter_func() to show/hide
- Fix searching for text which is not a valid JID

- Load contacts before calling show_all()
- Remove incremental load

The text block might contain a newline after the closing ```.
Remove it, so that `text.partition('\n')[2][:-4]` returns the expected
substring.

Fixes #10787.

Fixes #10788.

Fixes #10677

Fixes #10762