Enable translation for Win32 installer. French translation is provided.
New translation files need to be created in order to translate other
languages.
Translated strings are:
- strings displayed in installer pages, other than the NSI standard ones
(section names, section descriptions, installation types, VLC already
installed massage)
- context menu entries (Open With VLC, Add to VLC's playlist)
- name of actions displayed in Windows autorun dialog, for DVD and CD
(Play With VLC)

Add translation files for english and french (english.nsh and
french.nsh)
Add header file to declare translation variables (declaration.nsh)
Include declaration.nsh in NSI file
Replace every displayed string in installer by the variable defined in
language file.
Modify Makefile to copy translation files for Win32 installer

Language is chosen by NSI, with a dialog at the beginning of the
installer (which was already there).
Translated string are copied into the variables according to which
language was chosen. Default is English.
Signed-off-by: Christophe Mutricy <xtophe@videolan.org>