Add _postMeasure method to GUI Control

[pandaGaume]

Adding _postMeasure method to GUI.Control to allow derived classes take some post-measurement actions. This is necessary if we want to change a property such top, after the _measure() action but before the _computeAlignment() call.
Same should be obtained by overriding _measure() or _computeAlignment() into the derived class

 public _measure(): void {
      super._measure();
      this.top = this._i * this._currentMeasure.height ;
 }

but this will not prevent regression later (as it is defined as prefixed _ private method) while the intend of _postMeasure is clear and should be maintened to serve child purposes

so the above code may be replaced with

protected _postMeasure(parentMeasure: Measure, context: ICanvasRenderingContext): void {
     this.top = this._i * this._currentMeasure.height ;
}

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/14412/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14412/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14412/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/14412/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/14412/merge
https://gui.babylonjs.com/?snapshot=refs/pull/14412/merge
https://nme.babylonjs.com/?snapshot=refs/pull/14412/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/14412/merge#BCU1XR#0

[carolhmj]

LGTM but let's just check with @RaananW 🙂