Godot: Improve usability of curve edition in Path 3D

[rogeriodec]

Godot version:
3.1

OS/device including version:
Windows 10 x64

Issue description:
Unlike Path2D, Path (3D) has no curves, which forces us to draw curves manually via code using sin() and cos().

[eon-s]

Curve3D used on Path can be curved (try set in and out in code) but there are no handles to manage the in and out values of the bezier curve, it need an spatial gizmo.

Also, for precision, it may need this too #6914 but for the spatial version.

[JFonS]

[eon-s]

[rogeriodec]

[eon-s]

[rcorre]

[Calinou]