Add button that opens PDF manual

[izzyreal]

There are a couple of ways to do this.

1. The button is a URL pointing to the PDF. This likely results in multiple downloaded attempts. It might be a little tricky for the user to find the PDF in their file system every time they want to open it while using VMPC2000XL.
2. The PDF is downloaded by VMPC2000XL once. After that the feature "Open PDF manual" is fully offline available. Not sure what are the implications for export compliance in AppStoreConnect.
3. The PDF (2.3MB) is bundled with the portable binary, with the obvious downside that the binary becomes 2.3MB bigger.

1 is the easiest, because it wouldn't need to check if a local version is still up-to-date.