semibold -> bold in Color Picker headings

Change-Id: Ibd591ff43b0128eb8d7fb80468eb38ae68d187b2
Reviewed-on: https://gerrit.libreoffice.org/11328
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>