Add support for QSyntaxHighlighter TextEdit

Currently there's no way easily implement spell checking or syntax highlighting in TextEdit, similar to QTextEdit. An easy solution to allow users to subclass QSyntaxHighlighter and set the syntax highlighter in TextEdit, as a property.

For example:

TextEdit {
   ...

   syntaxHighlighter: MySpellCheckingSyntaxHighlighter { }

   ...
}

The text edit would automatically subclassed QSyntaxHighlighter's document to the TextEdit's internal QTextDocument.

Other option is to expose TextEdit's internal QTextDocument, as a property. The user could grab the QTextDocument, from TextEdit in qml, and pass to c++, where users can assign it to a subclassed QSyntaxHighlighter.

Adding a syntaxHighlighter option to TextEdit seems to be more straight forward. Users could still extract the TextEdit QTextDocument via the QSyntaxHighlighter.

Perhaps we should add both the document and syntaxHighlighter to TextEdit?