extract messages from umbrelloui.rc.cmake

Summary: Messages.sh searches for *rc files only an thus misses the menu items in umbrelloui.rc.cmake

Test Plan: running l10n-kf5/scripts/extract-messages.sh generates the missing messages in umbrello.pot

Reviewers: #umbrello, habacker

Reviewed By: #umbrello, habacker

Subscribers: habacker

Differential Revision: https://phabricator.kde.org/D9613