doxygen: Update pfb introduction.
wxgui: make dummy set_callback stub in nongl sinks.

Doesn't work, but also doesn't break the code.
digital: fixed some GRC bindings
docs: Added docs for packet transmission
digital: GRC fixes and cleanups for OFDM
Merge remote-tracking branch 'reynwar/master_pfb_docs'
Merge branch 'maint'
Merge branch 'master' into next