fixed behavior of Writer's Special Character dialog

before: the character is inserted at the end
after: The character will be inserted where the cursor is or will replace the selected string.

Change-Id: If5f7cac24c98a9105c38864da5a4c6363a35a834
Reviewed-on: https://gerrit.libreoffice.org/11642
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>