[FEATURE] Option to configure the fade-out time of the oscilloscope x-y trail

[PS2-4-3V3R]

I would like to be able to configure the time in which the path fades.
This is at low speed:

And this is at high speed:

As you can see, you can't distinguish anything when it goes fast.

It would even be fantastic to be able to optionally control whether or not to draw with a third channel.
This would better simulate a CRT, even if it is only two tones, off and on.

[esaruoho]

PR #240 adds a configurable Trail Persistence slider (0-50) in the scope properties dialog under the XY Plots section. Values 0-49 control the fade-out speed (lower = faster fade), and 50 gives infinite persistence (no fade). The default value of 2 matches the previous hardcoded behavior.