Make audio/video bit rates "properties"

[xhebox]

follow #731. This commit
completely removed all things in namespace bit_rate, and deprecated
functions are to be added back in another commit. set_xxx() is treadted
as a property of namespace audio&video, same as bit_rate change event.

toxav_basic_test is fixed, either.

As for adding legacy functions back, it should be finished in #732.

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This change is 

[iphydf]

Review status: 0 of 4 files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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toxav/toxav.api.h, line 479 at r1 (raw file):

     * @param friend_number The friend number of the friend for which to set the
     * bit rate.
     * @param audio_bit_rate The new audio bit rate in Kb/sec. Set to 0 to disable.

"audio_bit_rate" is now just "bit_rate".

See https://travis-ci.org/TokTok/c-toxcore/builds/334333799#L6208.

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toxav/toxav.api.h, line 525 at r1 (raw file):

     * @param friend_number The friend number of the friend for which to set the
     * bit rate.
     * @param video_bit_rate The new video bit rate in Kb/sec. Set to 0 to disable.

"video_bit_rate" is now just "bit_rate".

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Comments from Reviewable

[xhebox]

@iphydf that's too fast, i just saw that error...

[iphydf]

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Review status: 0 of 4 files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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Comments from Reviewable

[iphydf]

Reviewed 2 of 4 files at r1, 2 of 2 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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Comments from Reviewable