Make it possible to add mpl.rcParams to itself or deepcopy

Before it was not possible to set all rcParams to mpl.rcParams or use
copy.deepcopy on mpl.rcParams.

There were two problems:
1. validate_bool_maybe_none(None) was raising ValueError
   -> fixed by accepting this value
2. svg.embed_char_paths was deprecated by svg.fonttype but both have
   different valid values (bool vs strings).
   -> fixed by adding a "value translation" lambda to _deprecated_map
   which translates the boolean values to the string values of
   svg.fonttype.

Also added a new test for this: mpl.tests.test_rcparams.test_Bug_2543()