Allow opening BSP files by dragging over UI view

[tsa96]

Only looked at this briefly but I think it's possible (on Windows at least) (AvaloniaUI/Avalonia#2635). We want users, especially moderators reviewing maps, to be able to open BSP files very quickly to check contents - if someone's reviewing a dozen maps in one sitting we should make it very streamlined.

Best way I can think of is having dragging a BSP file anywhere onto the UI immediately open that BSP. If another BSP file (by file path) is already open, spawn a new instance of Lumper and open in there. Should be lightweight enough to handle that, though maybe worth checking someone doesn't accidentally drag 100 files onto it and spawn a 100 instances at once.