New and improved IK system for Skeleton2D

This PR and commit adds a new IK system for 2D with the Skeleton2D node
that adds several new IK solvers, a way to control bones in a Skeleton2D
node similar to that in Skeleton3D. It also adds additional changes
and functionality.

This work was sponsored by GSoC 2020 and TwistedTwigleg.

Full list of changes:
* Adds a SkeletonModifier2D resource
  * This resource is the base where all IK code is written and executed
  * Has a function for clamping angles, since it is so commonly used
  * Modifiers are unique when duplicated so it works with instancing
* Adds a SkeletonModifierStack2D resource
  * This resource manages a series of SkeletonModification2Ds
  * This is what the Skeleton2D directly interfaces with to make IK possible
* Adds SkeletonModifier2D resources for LookAt, CCDIK, FABRIK, Jiggle, and TwoBoneIK
  * Each modification is in its own file
  * There is also a SkeletonModifier2D resource that acts as a stack for using multiple stacks together
* Adds a PhysicalBone2D node
  * Works similar to the PhysicalBone3D node, but uses a RigidBody2D node
* Changes to Skeleton2D listed below:
  * Skeleton2D now holds a single SkeletonModificationStack2D for IK
  * Skeleton2D now has a local_pose_override, which overrides the Bone2D position similar to how the overrides work in Skeleton3D
* Changes to Bone2D listed below:
  * The default_length property has been changed to length. Length is the length of the bone to its child bone node
  * New bone_angle property, which is the angle the bone has to its first child bone node
  * Bone2D caches its transform when not modified by IK for IK interpolation purposes
  * Bone2D draws its own editor gizmo, though this is stated to change in the future
* Changes to CanvasItemEditor listed below:
  * Bone2D gizmo drawing code removed
  * The 2D IK code is removed. Now Bone2D is the only bone system for 2D
* Transform2D now has a looking_at function for rotating to face a position
* Two new node notifications: NOTIFICATION_EDITOR_PRE_SAVE and NOTIFICATION_EDITOR_POST_SAVE
  * These notifications only are called in the editor right before and after saving a scene
  * Needed for not saving the IK position when executing IK in the editor
* Documentation for all the changes listed above.

0 files changed, 0 insertions, 0 deletions