Add an 'About' popup

[Infernio]

Literally only opening this right now because I want to snag #666 :P

We should have an 'About' popup that informs users of:

• Who made this program
• WB's version (maybe this and the next one should be a separate one?)
• Versions of dependencies
• WB's license (including a note about AGPL from PyMuPDF if used)
• Third-party licenses (the whole reason I made that file machine-readable ;))
• And a nice big logo
• Maybe make this tabbed? One tab for credits, one for licenses and one for versions?

This is very simple, mostly just some GUI design. Still, I want it to show up on the changelog so an issue is needed.

[Arthmoor]

I recommend this be added to the start of a proper "Help" menu since that's where most "About" screens are linked from in other things. Definitely a good call though. The things we never think of until we do :P

[Infernio]

Yup, this will be the start of the 'Help' menubar item. I have some plans for interactive help in WB too, but that's far off.