Meta Base invalidate() should not require instance context

[kitsonk]

Enhancement

As noted in a PR, the meta Base calls an invalidate() passed in during the construction properties, but the way it calls it requires its context to be the instance:

widget-core/src/meta/Base.ts

Lines 39 to 41 in 72316bc

	protected invalidate(): void {
	this._invalidate();
	}

When trying to invalidate a meta instance though from within something like an event handler, it is a common situation that the protected method will lose its context. This requires downstream meta implementors to statically bind to the proper context to call a passed property that is already statically bound from the implementing widget. We should find a way to ensure that the Base.invalidate() works without having to statically bind to the instance.