Begins an interactive move operation.
You might use this function to implement draggable titlebars.
| this |
a `GdkToplevel` |
| device |
the device used for the operation |
| button |
the button being used to drag, or 0 for a keyboard-initiated drag |
| x |
surface X coordinate of mouse click that began the drag |
| y |
surface Y coordinate of mouse click that began the drag |
| timestamp |
timestamp of mouse click that began the drag (use [[email protected]_time]) |