fdo#60664 HIG fixes for Thesaurus dialog

* Disable manual resizing
* Standardize column width to 12
* Reduce a widget’s size request a little
* Add missing colons to labels that precede controls (fdo#78826)
* Unrelated tweaks to the XML are courtesy of Glade.

Change-Id: I6d746b909adebde284e1848417373dd8b06e75fe
Reviewed-on: https://gerrit.libreoffice.org/11992
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>