X11: introduced a TfpgX11Drag class with handles most of the XDND messages
X11: introduced a TfpgX11Drag class with handles most of the XDND messages

We offload most of the DND message processing to the TfpgX11Drag class. This
helps keep the functionality with a clean design.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI