To support drag-and-drop for LibWeb, the drag events need the full mime
data from WindowServer, not just the list of stringified mime types. We
only provided the full data in the drop events. This patch provides the
Core::MimeData type, as well as other mouse-event information needed for
LibWeb (what buttons / modifiers are pressed).
To do this, we add a specific WindowServer IPC to inform the Application
of the event. We previously tagged mouse-move events with some drag data
and formed the drag event client-side. That extra data is removed in
favor of this new IPC.
