Fix copy and pasting image from clipboard

If the QMimeData contains an image, it actually has a mime type of
application/x-qt-image. At least in some cases accessing the image/*
data returns a 0 length array. Accessing the data via ->imageData works
however. So we use that as our accessor and pass it to the preview
dialog.

src/TextInputWidget.h

@@ -34,10 +34,6 @@
 #include "popups/ReplyPopup.h"
 #include "popups/SuggestionsPopup.h"
 
-namespace dialogs {
-class PreviewUploadOverlay;
-}
-
 struct SearchResult;
 
 class FlatButton;