Added Active control, fade controls, alpha control, and updated documentation

[Mikeysax]

This PR adds:

• The ability to activate and deactivate a trail
• The ability to control the whole trail alpha via single parameter
• The ability to trigger the the whole trail to fade in or out based on time when activated or deactivated
• The ability to fade out the trail based on each individual trail particle lifetime
• Updated documentation describing the above parameters added as well as missing mask and mask_strength properties

[Mikeysax]

@celyk Let me know if there is more to do here to get it merged?

[jmschrack]

@Mikeysax
Small suggestion: change the default hint of your fade_curve to hint_default_white , as currently it's defaulting to black, resulting in the GPU trail not working with default settings.

Otherwise it gives the impression that the GPUTrails are broken out of the box given the cantankerous reputation that Godot's GPU RibbonMesh particles have. :)

[Mikeysax]

@jmschrack Awesome; thanks for catching that. I just fixed it.

[Mikeysax]

@celyk Anything else needed to get this merged?