Rename color picker button and add an icon to it

Change-Id: I24def42d6167c6e7a7a4d5871f2a6e3b51230d58
Reviewed-on: https://gerrit.libreoffice.org/12086
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>