implement SailfishOS support, made in collaboration with vakkov

sorry for building your hopes up, it currently segfaults:
https://bpaste.net/show/319c346897f6
i hope someone can help me out there

i've continued what vakkov started with cleanups and i tried to make it so that other platforms still work (but i cannot test all of them :()
This should be done in `platform/common/sdl_mixer_impl.h` instead and `apkenv_mixer_load_music_buffer` (from `mixer/mixer.h`) used for doing loading of music.