Revise session API to allow for multiple sessions in future.
Move public headers into include/libsigrok.

This is needed so that the C++ bindings, the header for which
references "libsigrok/libsigrok.h", can have a valid include
directory passed to build them before the headers are installed.
SWIG: Remove obsolete lowlevel wrapper methods for sr_output_format.
SWIG: Rename libsigrok.i to lowlevel.i.
Change prototype of sr_trigger_new to take const char *.
Add C++ bindings.
Add new generic SWIG bindings based on C++ classes.
Reimplement high-level Python bindings on top of SWIG/C++ bindings.
Add Java bindings.
Remove obsolete low-level language bindings.
configure: Deal with missing macro a bit better.

The AX_CXX_COMPILE_STDCXX_11 macro is found in the autoconf-archive
package. If this is not installed, invoking the macro spits out an
error message that makes it look like a syntax error. This wraps it
in a check.
configure: Add C++ bindings dependency checks.
configure: Fix java --enable check.
Fix building Python bindings with pygobject >= 3.7.91.
Suppress some SWIG warnings that don't matter.
Pass configured prefix to python setup.py install step.
Assorted build system fixes for C++/Python/Java bindings.
Additional build system fixes for C++/Python/Java bindings.
bindings: Filter out various pygccxml noise.
configure: Add --enable-bindings switch.