qtgui: added ability to clear waterfall plot data from GR block.
qtgui: added auto-scale checkbox to time plot.
qtgui: adding features to QT-based time plotters.

Made parent class to make updates to all types easier.
qtgui: wip: working on improving PSD plotters.
qtgui: making an even more generic set of plotting tools.
qtgui: updated PSD plots with base class to more easily expand.
qtgui: improved PSD plotting tools to use more complete and complex base classes.

Some tweaks to time plotters for better GUI interactions.
qtgui: waterfall plots can take multiple signals.

Signals are overlaid with an alpha (transparency) value for each layer.
qtgui: make default nconnections in qtgui plots 1.
qtgui: expose set_intensity_range for spectrogram gui to GR block.
qtgui: psd plot cleanup.
qtgui: updated constellation and spectrogram plotting tools.
qtgui: added ability to toggle time plots into stem plots.
qtgui: added checkbox to time plotting apps to toggle stem plot on/off.
qtgui: added method to disable right-click menu.

Disabled menu in plotter tools to not conflict with states between C++ and Python domains.
qtgui: Major changes to qtgui sink interfaces to allow more tunable control through GR blocks.
qtgui: Updated plotting tools for more control through the GUI interface.
qtgui: fixed waterfall update time to be related to the sample rate.
Merge branch 'qtplotters' into next
qtgui: fixed stem plot marker style update.