qtgui: implement fixed frame rate scrolling for waterfall sink

[noc0lour]

The waterfall sink currently uses a crutch to limit the frame rate of the display called "update period" and it literally does leave out a whole lot of frames from the display and thus loses a lot of information.

Instead of doing that a fixed frame rate can be calculated based on sample rate and number of bins and the data can be aggregated in a separate buffer and then displayed in a smooth scrolling instead of spitting out everything received from upstream.