Reads thumbprint of desired X509 certificate from file

[jimmyswimmy]

I wanted to be able to not always have to select the X509 certificate when I start Keepass. I'm not a C# programmer (or .net, man there's a lot in there) so this could be a lot cleaner I'm sure; it's built from stackoverflow and ms.com snippets.

Anyway, this reads a "X509_thumbprint.txt" file in the main keepass directory, if it exists. If so, it reads the thumbprint (the first/only line of the file, probably this won't work if there's a CR/LF, I didn't care) from the file, and searches for that.

Probably this gives weird behavior if there IS such a file and no perfectly matching certificate. I didn't test for that.

Also, this is the first time I've ever attempted any kind of pull request, not sure what it's supposed to contain. Use it if you want it, ignore if you don't.