Added PreviewTextInput event with WPF implementation.
Added text input sample.
Merge branch 'master' of github.com:mono/xwt into text-input
[WPF] Removed support for default text foreground, which was interfering with region foregrounds.
[WPF] Use PreviewTextDown/Up instead of normal TextDown/Up so that events such as [Up] key press are detected.
Merge pull request #326 from luiscubal/text-editor

XwtPlus.TextEditor-related changes