fpicker module TCHAR elimination and A/W clarity

NB fpicker was already being compiled as UNICODE

Use SendMessageW() as appropriate for clarity

Use SAL_N_ELEMENTS for buffer capacity (number of WCHAR) when retrieving class name

Change-Id: Ie6d0b1df1b9a2fb18f4cdfc51e5d4c3c55c9ca17
Reviewed-on: https://gerrit.libreoffice.org/26526Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>