EditableText change detection using parley::Generation

[ickshonpe]

Objective

Improved change detection for EditableText.

Solution

• Remove the text_edited field from EditableText that was used for manual change detection.
• Use the PlainEditor's Generation to track changes.
• New component EditableTextGeneration. Newtypes parley::Generation. Stores the generation from the last TextLayoutInfo update.
• If EditableText::editor's and EditableTextGeneration's generation values aren't equal, reupdate TextLayoutInfo.
• Added support for TextLayout::justify.
• Split up editable_text_system into two systems update_editable_text_styles and update_editable_text_layout. The text input's style values need to be updated before layout, so the measure func returns the correct size for the text layout.
• New EditableText testbed_ui scene.
• Added two numeric inputs to multiline_text_input that allow you to set the height of the multiline input and its fontsize.

Testing

cargo run --example multiline_text_input

The cursor appears to be missing for the numeric inputs in the example but it isn't. The cursor gets clipped because it's at the end of the right aligned input value text. If you press left it comes into view. Cursor and scrolling behaviour needs some adjustments but that's out of the scope of this PR.