Incomplete set of output parameters for mp3 coders

[TeoFilin]

This converter has the fastest mp3 coders among media-converters at Google Play.
Still the choice of encoding params seems strange:

• Libmp3lame is VBR-only (why?! CBR/ABR is usual choice in Lame-MP3; VBR gives artifacts with low bitrate, especially w/o resampling);
• Libshine is CBR-only (OK for speed), and the slider starts with 45 kbps - which causes the coder to fail (acceptable minimum is 48 kbps);

• the slider isn't appropriate control for bitrates, since they have discreet set of possible values, which are hard to target precisely (48, 56, 64, ... kbps) - combo-box is better;

• mp3 coders must support much lower bitrates (I prefer 32 kbps CBR, mono 16 kHz - as compact + compatible + tolerable quality {for speech});
• lower bitrates of mp3 require resampling (44 kHz -> 16 kHz for 32 kbps mono) - else nasty artifacts appear; some codecs seem to fix this automatically, but maybe here we need manual control for sampling rate.

[simonpham]

TODO:

• Replace slider with combo-box with a discrete set of bitrate
• Add 32 kbps options
• Add Settings option to enable Advanced mode, which gives control of CBR/ABR, resampling,...