.ui tweaks to PDF export tab pages

Added missing colons, use sentence case for check boxes [1], allow
descriptive security labels to wrap, add 6 px padding for the Sign
page.

Change-Id: Iafc0788e367ab74aa0ec740b06892926b0b00390
Reviewed-on: https://gerrit.libreoffice.org/11408
Reviewed-by: Katarina Behrens <bubli@bubli.org>
Tested-by: Katarina Behrens <bubli@bubli.org>