HIG fixes for smath dialogs

Disabling manual resizing, setting modal flags, fixing spacing
and adding missing colons to labels that precede controls.

Change-Id: Id0f180355d302206a632af97dafb6a391c8b4472
Reviewed-on: https://gerrit.libreoffice.org/12105
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>