Doxyfile: Various additions/improvements.

 - Show a short project description, the libsigrok version (x.y.z or
   "unreleased development snapshot"), and the sigrok logo in the
   Doxygen output.

 - Ignore a bunch of files and directories which don't contain public
   libsigrok API anyway.

 - Remove the SR_API prefix from the Doxygen output (all
   functions/symbols in the output are part of the public API anyway).

 - Various changes of settings to get nicer and more useful output.