File drag&drop

[dhardy]

File hover/drop:

• determine coordinate (may be a little tricky since winit doesn't communicate whether the input source a mouse or touch event)
• send FileHovered and FileDropped to the appropriate widget (including when moved over a new widget)
• support grabs (include source to allow use of Manager::request_grab)

Currently, winit only reports HoveredFile once on hover-over-window start, HoveredFileCancelled when leaving the window, and DroppedFile when releasing. These include a path but nothing else. Additionally (at least on X11), the usual cursor and touch events are not received. It is therefore not possible to implement this over the current winit API. See rust-windowing/winit#1550

[dhardy]

Note: the cursor position should be received immediately after a drop ends which could be used to target the drop to a specific widget. This is hacky and incorrect for touchscreen drops (on platforms other than X11).