HIG: Spacing and labeling fixes for Outline Numbering dialog

* Missing colons in labels that precede controls (fdo#78826)
* Sentence capitalization for non-headings
* Make dialog modal and non-resizable for users
* Standardize spacing values to 6 and 12
* Remove extraneous 10-px paddings in tabpages
* Tweak a label to remove a silly “<->”

Change-Id: I4dabad2ec4a610ff3f55e31e078f465a7e42c8f2
Reviewed-on: https://gerrit.libreoffice.org/11756
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>