More options for File Dropper

• Added 5 more events - file_drag_enter, file_drag_over, file_drag_leave, file_drop_start, and file_drop_end.
• Added 8 more functions to allow/deny drop and change the drop cursor.
• Added position and mouse/modifier key flags to the events.
• Dragging a zero-file selection (e.g. a snippet of text) to the game no longer allows drop.

This gives you a bunch more flexibility with where to allow drag operations and providing visual feedback to the user.