New help window

[mole99]

This PR replaces the help window with links pointing to the repository and online documentation.

While it was nice to have a documentation built into CACE, the online documentation has many benefits:

• It's easier to navigate
• You can have multiple tabs open at once
• The documentation can fully utilize Markdown
• We don't have to sync offline and online documentation anymore
• Less code to maintain
• Also possible to host offline

Note: Merge #45 first, then I will rebase this PR Rebased.

[mole99]

This is what the Help Window looks like now:

It contains the version string, a short description and links to the repository and the documentation.