Minimal additions

I've changed some things which I believe may be useful:
- allow multiple files to be uploaded from the file upload button (cff2756)
- display a warning box when hotkeys couldn't be registered, as this is pretty much the core functionality of the tool, I think an invasive warning is warranted (669b542)
- bugfix: regex for filename sanitation had a misplaced group negation (84986e7)
- asynchronously check for updates; using the Form.Shown handler is not a good place for starting async WebClients either because the wndproc is blocked while proxy information is retrieved, hence the extra Task.Run (cff2756)