Allow hitting 'ESC' to cancel a file drag operation

[frankmilliron]

Modern macOS allows users to hit Escape to cancel a file drag operation while the mouse is down. In A2D, you have to drag the items back into the original folder instead to cancel, once selected files have been moved out of the original folder.

[inexorabletash]

This is tricky - MGTK doesn't check for keyboard events while the mouse is down:

a2d/mgtk/mgtk.s

Line 5228 in 5845045

bcs end ; minus = is down

If that were to change, then the fix would be in toolkits/icontk.s circa

a2d/toolkits/icontk.s

Line 847 in 5845045

not_drag:

• call XdrawOutline (same as today)
• if highlight_icon_id is non-zero, call UnhighlightIcon (same as today, but don't jump away yet...)
• GetEvent (so it's consumed)
• If event_kind is EventKind::key_down, then set highlight_icon_id to 0 (to cancel) and exit via L9C63
• otherwise, if highlight_icon_id is non-zero, exit via L9C63 (same as today)
• otherwise, same logic as currently

[inexorabletash]

As a workaround, you can drop the files on an empty spot on the desktop.

[inexorabletash]

Just bypass MGTK. Not too bad.