Add BEZIER_CURVE_SUPPORT — G5 command

[thinkyhead]

Adapted from https://github.com/giomasce/Marlin/commits/Development by @giomasce and discussed in MarlinFirmware/MarlinDev#417

• G5 command supporting bezier curves in the XY plane
• Minor cleanup of configurations, Marlin.h

[jbrazio]

As self-contained by BEZIER_CURVE_SUPPORT which is disabled by default I vote for inclusion on RC. 👍

[thinkyhead]

I just put it together so haven't had a chance to test it. I'm not sure when I will, perhaps tonight. If it works well I will merge it.

[Grogyan]

Which slicers produce G5 gcode?

[giomasce]

@Grogyan The only programs that produces G5 codes that I know so far is mine Inkscape plugin https://github.com/giomasce/thlaser-inkscape-plugin that I use for my laser cutter (it is usable and I actually use it, although it is still rather alpha). However, given how much it is easier to produce G5 codes instead of having to do that terrible biarc interpolation, I bet that other Gcode produces for laser cutters might be interested in using it.