Return DRAG_ABORT on UnmapNotify from drag_pointer

Return DRAG_ABORT on UnmapNotify from drag_pointer

Add DRAG_ABORT to enum drag_result_t. DRAG_ABORT will indicate the drag
operation cannot be completed.

Return DRAG_ABORT on UnmapNotify, or when the keyboard or pointer cannot
be grabbed.

Add DRAGGING to return value for drag_result_t. DRAGGING is used
internally by drag_pointer to indicate the drag is in progress.

Change DRAG_CANCEL to DRAG_REVERT to clarify the distinction between
"abort" and "revert/cancel" actions.

Fixes an issue that caused i3 to crash when a user is dragging or
resizing a floating window that becomes destroyed.