Support keyboard-locale-aware input in QTextDocument-based editors

Qt currently lays out text based on the unicode bidirectional algorithm. This can cause unintuitive behavior in some cases, such as when you embed the Spanish text ¿QUE? inside a RTL paragraph. The first character of the Spanish text is a neutral character in unicode terms, which will get its directionality from the paragraph instead, giving the two question marks the opposite of their intended order.

A solution to this is to implement the editor so that the keyboard input language has an effect on the directionality of neutral characters. Qt should support this solution under the hood, by allowing more control over the bidi algorithm for the editor.

The solution should also maintain this across export/imports, to ensure that the layout of the document can be properly saved. This can be done with unicode directionality marks when exporting to plain text, or directional attributes when exporting to HTML or other rich text formats.