wxgui: make dummy set_callback stub in nongl sinks.

Doesn't work, but also doesn't break the code.
wxgui: fix scaling issue of nongl FFT plot (issue #523).

wxgui: add some noise to the fftsink_nongl test case.
Merge branch 'maint'
blocks: fixed missing install of file_sink.h.
blocks: more missing insalled header files.