qtgui: timing is handled in the blocks; removing the qt timer.

This update dramatically reduces the X server overhead of the QT sinks.
qtgui: fixed call to _a version of volk kernels for waterfall plots.

(same fix tha was applied to freq_sinks in bf6d843ebde3a36).
grc: fix error message in console when using qtgui blocks
filter: set fft and ifft pointers to NULL not 0 in constructor.

Addresses CID 1189412: Explicit null dereferenced (FORWARD_NULL)
runtime: setting proper default value for log_level if no prefs file is found.