Fix issue where a layout manager's NSTextStorage could be `nil` at initialization
but later set by introducing the `_setTextStorage:` method to NSTextView.

Add stubs a for missing properties in NSTextView.

Refactor `firstTextView` to properly find and return the first text view,
or `nil` if there are no text views.

Use 64-bit ready types where applicable.

Relocate unrelated code from _setTextStorage

Adds code in `initWithData` to infer document format for RTF, HTML, and plaintext documents.
Additionally refactors NSAttributedString to use the correct attribute types as described in Apple's documentation.

…ocumentation

Add stubs for content size, frame size, and magnification size,
and add basic implementations where feasible.

Implement NSTextTab handling based on the GNUStep implementation,
with formatting and other adjustments.

…aints`

…r reference.

…oFitRect

…tMagnfication through rect and point, and allowed negative values to be passed to scaleUnitSize