[1/3] riched20: Implement handling of WM_KEYDOWN VK_RETURN in a dialog mode.

[Sergey Khodych]

> 1. editor->hWnd may be NULL for windowless richedit controls, but you
> code doesn't seem to take that into consideration.
> 2. The style flags that you get from GetWindowLongW should probably
> come from the cached values in editor->styleFlags.
The main reason why GetWindowLongW is used there is that 
editor->styleFlags actually isn't window style and doesn't contain 
ES_WANTRETURN. What's the proper way to get actually window style in 
this case?