Implement TextEditor

[stackotter]

This PR introduces a TextEditor control for editing multi-line text. For now it's borderless (i.e. has all platform-specific 'text field' looking styles disabled) to allow for it to be used in the widest variety of use cases.

The text editor background in the NotesExample app (see screenshots below) was added using the background modifier and doesn't come with the text editor (unstyled by default).

Other changes

• Added default Edit menu items to AppKitBackend's default menu to enable keyboard shortcuts such as cmd+a, cmd+c, cmd+v etc to be used in text inputs.
• Fixed background modifier's propagation of idealHeightForWidth and idealWidthForHeight (previously just set to the child view's idealSize.x and idealSize.y)
• Fixed ScrollView's sizing behaviour (to a certain extent; just enough to fix NotesExample)

Known issues

• GtkBackend and Gtk3Backend overestimate the required height of text editors by about half a pixel per line of text (or less) on macOS (but work fine on Linux).

Future directions

• A textEditorStyle modifier to allow platform-specific decorations to be enabled (so that text editors can fit in with text fields for form-like use cases)

Screenshots

AppKit

WinUI

Gtk 4

Gtk 3

The missing border radius is a known issue for Gtk3Backend.

UIKit

[stackotter]

Will fix these when I'm back at my laptop, then I'll merge.