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.
Add C++ bindings.
Reimplement high-level Python bindings on top of C++ bindings.
Add Java bindings.
Remove obsolete low-level language bindings.