qtgui: adds ability to set the graphics style rendering of the qtgui sinks.

Set through gr-qtgui.conf prefs file. Documented in QT GUI section of manual.
qtgui: fixed call to _a version of volk kernels for waterfall plots.

(same fix tha was applied to freq_sinks in bf6d843ebde3a36).
Merge branch 'maint'

Conflicts:
	gr-qtgui/lib/waterfall_sink_c_impl.cc
	gr-qtgui/lib/waterfall_sink_f_impl.cc

Conflict due to white space issues.