Add struct kas::text::ConfiguredDisplay; replace trait Editor with struct

[dhardy]

This adds a new abstraction: kas::text::ConfiguredDisplay is somewhere between TextDisplay and Text in that it contains the TextDisplay, TextClass, FontSelector, Rect, Status etc. but not the actual text (or the text formatter).

The text editor Component uses ConfiguredDisplay instead of Text internally, allowing the highlighter to be moved out of struct EditorComponent. This removes the need for generics here, and thus the motivation for using a trait, allowing a revert of #676 (motivation: simplicity).

EditBox, EditField and dialog::TextEdit now all deref to Target = Editor (non-mut only) and have fn edit(..) supporting usage of &mut self methods on Editor (this allows correctly calling EditGuard::edit and Component::prepare afterwards).

Basic text-drawing fns (using kas::theme::Text) have been removed from DrawCx (replaced with methods on Text).

kas::theme::Text is moved to kas::text::Text.