XwtPlus.TextEditor-related changes

This is the full set of changes that XwtPlus.TextEditor (see https://github.com/luiscubal/XwtPlus.TextEditor) requires to run properly. It is WPF-only because the text input event is not implemented under GTK nor OS X.

This fixes:
1. Issue #301
2. Issue #274 (though it properly adds new issues, considering what it does is disable the default text foreground brush)

This also includes #298.