fix callback typedefs and declarations

fix callback typedefs and declarations

Remove declarations of callback and extension_callback as these are
functions declared in header but not defined anywhere.

Also rename typedefs callback_function and extension_callback_function
to lws_callback_function and lws_extension_callback_function so all
symbolx exported by header have lws prefix;

Signed-off-by: Denis Osvald <denis.osvald@sartura.hr>