Convert tp_Trendline.src to .ui

Change-Id: I429a90441a68a94403d89d7746628528ed4dc876
Reviewed-on: https://gerrit.libreoffice.org/6638
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>