Xwayland: support proxying drag and drop operations to the Wayland compositor

It is possible for drag and drop to drop data on a window that is currently covered by a different window. To reproduce this:

1. Open two text editor windows (such as Mousepad) using Xwayland.
2. Cover one (but not the other) with a native Wayland window, such as weston-terminal.
3. Drag and drop text from the uncovered one to where the covered one is.

This causes text to be dropped into a hidden window, which should not happen. I tested this with Weston.

To fix this, Xwayland can intercept drag and drop operations and proxy them through the Wayland compositor.

Combined with #1649, this could allow a full-featured rootless unprivileged Xwayland.