Introduce a version manager

[alepouna]

I have noticed a lot of users in the Discord keep having errors that are fixed in newer version of the farmer. It will be helpful for everyone if there was a system in place to check the current app version and the latest one on GitHub and offer a warning message that they are running an old version of the program.

Ideas: We can have a file in this repo named "version.json" or similar so that the app can do a fetch request and match the variable in the code. If online version is higher than the stored one, we can show a big fancy warning to the user that they are a version behind.

[LeagueOfPoro]

Or we could do a complete version manager (downloading new versions). That would be a bit more complex though.

[MrBartusek]

Fixed by #80