fdo#72338: startcenter: Show busy mouse pointer when starting apps.

Change-Id: I8f254419ea7bb4e3e9835bd9fab8229ed89cd0e9
Reviewed-on: https://gerrit.libreoffice.org/7828
Reviewed-by: Joren De Cuyper <jorendc@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>