Add 'About' Dialog to top-menu

[udlose]

It would be helpful to users if they had a way to navigate back to the https://github.com/udlose/MermaidPad home page and see contributors and 3rd party notices.

- Display a clickable link in the top menu
- Link should be right-justified

• Create Menu item on the new menu (added in develop here - https://github.com/udlose/MermaidPad/blob/develop/Views/MainWindow.axaml
• Menu item named About
• It has no submenu items (@petith-au unless you can think of something here that would might change this to a Help menu item and then have an About sub-menu)
• Link url should be https://github.com/udlose/MermaidPad
• dialog should contain list of 3rd-party libraries used - see Third Party Libraries
• I don't think we need a scrollable TextEditor (read-only) here unless the info to put in the dialog makes the dialog way too big. Otherwise, add a read-only TextEditor
• We can add the "Check for updates" after we get the Velopack installer in place - see Create Installer for distribution on Windows #144

"loosely" based on this style - @petith-au if u want this one, I leave it to your artistic abilities lol

[petith-au]

Created a draft PR #184

[petith-au]

Yeah, I can definitely jump on this change now. I will update my PR this week.

[udlose]

@petith-au I Did you want to get this in to 1.2.0? I can hold off on creating the release, or we can push it to 2.0.0 - nbd

[udlose]

moving this to 2.0

[udlose]

@ @petith-au, @Mustansir-06 has added the Help menu that you can piggyback on for this change. All changes are merged into develop