Remove the setting of a fixed size from the envelope graph and only set a minimum size.

Adjust the painting code of the envelope so that it does not assume fixed widths and heights anymore.

The new code is more or less divided into two parts. The first part calculates `QPointF` instances for the different points. In the second part these points are then used to draw the lines and markers. This makes the actual rendering code much more straight forward, readable and maintainable.

The interpolation between the line color of an inactive and an active envelope has also been restructured so that it is much more obvious that we are doing an interpolation in the first place. The colors at both ends of the interpolation are explicit now and can therefore be adjusted much easier. The actual color interpolation is done in the new helper function `interpolateInRgb` which might be moved elsewhere later if needed.

The line is rendered as a polyline instead of single line segments.

The drawing of the markers has been abstracted into a lambda (with some outside captures though) so that it can be easily adjusted if necessary. The markers are rendered as circles instead of rectangles because that looks much nicer especially when the widget is rendered at a larger size.

The width of the lines and marker outlines is determined using the size of the widget so that it scales with the size.

A `lerp` function has been added to `lmms_math.h`.

Add the two new scaling modes "Dynamic" and "Relative" so that there are three modes now:
* "Dynamic": This modes corresponds to the rendering strategy of the previous implementation. Initially 80/182 of the available width is used as the maximum width per segment. This can be interpreted like a "zoomed" version of the absolute mode. If the needed space becomes larger than the full width though then it falls back to relative rendering.
* "Absolute": Each of the five segments is assigned 1/5 of the available width. The envelopes will always fit but might appear small depending of the current settings. This is a good mode to compare envelopes though.
* "Relative": If there is at least one non-zero segment then the whole width is always used to present the envelope.

The default scaling mode is "Dynamic".

Add a context menu to the graph what can be used to set the current scaling mode.

Move the function `interpolateInRgb` into the new class `ColorHelper` as it will later also be needed when the LFO graph is made scalable.