Previously using EBU R128 forced the use of the bs1770gain backend.
This change adds a whitelist of backends supporting R128. When the configured
backend is in that list it will also be used for R128 calculations. Otherwise
bs1770gain is still used as a default.

This should not change the overall behaviour of the program at all, but allow
for further R128-supporting backends to be added.

Add replaygain backend using ffmpeg's ebur128 filter.

The album gain is calculated as the mean of all BS.1770 gating block powers.
Besides differences in gating block offset, this should be equivalent to a
BS.1770 analysis of a proper concatenation of all tracks.

Just calculating the mean of all track gains (as implemented by the bs1770gain
backend) yields incorrect results as that would:
- completely ignore track lengths
  - just using length in seconds won't work either (e.g. BS.1770 ignores
    passages below a threshold)
- take the mean of track loudness, not power

When using the ffmpeg replaygain backend to create R128_*_GAIN tags, the
targetlevel will be set to -23 LUFS. GitHub PullRequest beetbox#3065 will make this
configurable.
It will also skip peak calculation, as there is no R128_*_PEAK tag.

It is checked if the libavfilter library supports replaygain calculation. Before
version 6.67.100 that did require the `--enable-libebur128` compile-time-option,
after that the ebur128 library is included in libavfilter itself. Thus we
require either a recent enough libavfilter version or the `--enable-libebur128`
option.

Add changelog entry for the new ffmpeg replaygain backend.

Use keyword arguments to make the ffmpeg parser more readable.

This commit mostly addresses feedback:
- remove some unused parenthesis
- fix a typo
- expand some docstrings
- document that ffmpeg is usually easy to install

Use the POSIX character class instead of `\s` to match all whitespace in a
regular expression describing the language of valid inputs, in order to avoid a
test failure for the invalid escape sequence `\s` in Python strings.