new Waveform & Spectrum component + relative coords

[tassaron]

• New "Waveform" component with modes for using the FFmpeg filters showwaves and showfreqs
• New "Spectrum" component with modes for aphasemeter, showcqt, ahistogram, showspectrum, avectorscope
• Components can define "relative widgets" which will have their value stored normally (pixel value as an integer), but this value is also stored separately as a float which is used to update the pixel value in the widget when the output resolution changes. Thus a title text in the centre of the screen will remain in the centre of the screen. Unfortunately this breaks presets and projects saved in old versions.
• Text component has a 'center' button to set the x, y to 0.5, 0.5
• Components can define "color widgets" which will have their value stored as an rgbTuple instead of a string, update the associated pushButton stylesheet, and automatically create a pickColor method. So adding a new colour option to a component is now just as easy as adding a new checkbox.