dict: const pointers to dictionary where possible

This avoids temporaries or ugly casting in the calling code where
const dictionaries are used. Esp. helpful when writing C++ wrappers
for an AVDictionary having const member functions and CTORs with const
references.

Signed-off-by: Roman Fietze <roman.fietze@telemotive.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>