LibGUI+WindowServer: Make DragOperation hold a MimeData instance

...instead of maybe bitmap + a single mime type and its corresponding data.
This allows drag&drop operations to hold multiple different kinds of
data, and the views/applications to choose between those.
For instance, Spreadsheet can keep the structure of the dragged cells,
and still provide text-only data to be passed to different unrelated editors.

Libraries/LibGUI/FileSystemModel.cpp

@@ -403,7 +403,7 @@ Variant FileSystemModel::data(const ModelIndex& index, ModelRole role) const
         return node.full_path();
     }
 
-    if (role == ModelRole::DragData) {
+    if (role == ModelRole::MimeData) {
         if (index.column() == Column::Name) {
             StringBuilder builder;
             builder.append("file://");