Add config key for amplitude.

This is intended for setting (or getting) the amplitude of a source
which doesn't really have an MQ associated with it, such as the demo
driver's analog channels.
demo: Support changing the amplitude of analog channels.
output: Rename instance private storage pointer to priv.

This makes it consistent with other libsigrok fields used for this purpose.
demo: Attach analog generator to channel, not channel group.

This also adds a new channel group "Analog", which has all analog
channels in it.
output/wav: Initial module skeleton.
output/wav: Initial module implementation.
output/analog: Fix channel deinterleaving.
output/wav: Fix channel deinterleaving.
input/wav: Deal with IEEE float samples in WAV files.

This also skips chunks before the 'data' chunk in WAV files, as
this is quite common.
input/csv: Match format on .csv extension in filename.
input/vcd: Make less noise in format match failure.
Doxygen: Update paths.
C++ bindings: Reimplement enums.py using doxygen XML output instead of gccxml.
C++ bindings: Attach documentation to enum wrapper classes.
bindings: Transfer C++ documentation strings to Python and Java wrappers.
python: Add documentation generation.
java: Add documentation generation.
C++: Add parameter documentation and additional method descriptions.
bindings: Transfer C++ method parameter documentation to Python/Java bindings.