its a (very) expensive operation.

Don't call it ~20 times on startup, instead call it once when the user
actually opens the styles dropdown.

(cherry picked from commit e8bcffc7)

Change-Id: I7bb28d0739bf917d23041f384f60a39a7ed5cbea
Reviewed-on: https://gerrit.libreoffice.org/20349Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>