Add DxfDocument.LoadAsync

[bvirkler]

Is there a reason there isn't an async version of the Load method? If not, would you be open to a PR adding this?

[haplokuon]

No, no special reason, I haven't even thought about it, I just know the basics of parallel programming. In this kind of cases, where a long process is executed, I just create a new thread to avoid locking up the UI. You can try if you wish, while it does not modify too much the loading and writing process I am fine with it, I will still need to look at the changes, thought.