Add copyleft symbol, capital schwa, open O and open E

This merge request is only to add a couple symbols that are used in the International Phonetic Alphabet, and variants that, as far as I know, are used in some African languages.

I tried to design the mnemonics following patterns already in the Compose file — I'm not sure if you prefer others. Both open E and open O start by the depiction of the lowercase variant in both X-SAMPA and Kirshenbaum.

Maybe in the future we could add other IPA signs in Compose? I'd rather ask before adding them in masse.