qtgui: fixed problem with setting fft size and averaging of the float waterfall at runtime.
qtgui: allow toggle to only show positive half of waterfall spectrum.
qtgui: fixed comment.
qtgui: added triggering to freq sinks.

- Trigger off power (norm/auto): if any power bin in the FFT is greater than the level, output.

- Trigger off tag: if the given tag is found, take FFT from that triggered sample.

- Simplifies work funciton by relying on set_output_multiple so we always work off a multiple of FFT-length slices.
qtgui: removes use of outputsize in freq sinks.

- Was a bit confusing and caused use of different FFT sizes than the user specificed; often resulting in 1 more than a power of 2 for standard values (would be slightly less efficient).

- The problem was more in the _resetXAxisPoints calculation; I think this gets the desired effect of the frequency plots.