Add version number to About page

[Pohli]

See title.

[Tomin1]

For reference, this is how Patience Deck does it:
https://github.com/Tomin1/patience-deck/blob/cb5af775dc39a1d2c645646de7e8a31fc184f3fe/rpm/patience-deck.spec#L47
https://github.com/Tomin1/patience-deck/blob/cb5af775dc39a1d2c645646de7e8a31fc184f3fe/src/patience-deck.cpp#L84
https://github.com/Tomin1/patience-deck/blob/cb5af775dc39a1d2c645646de7e8a31fc184f3fe/src/src.pro#L14
https://github.com/Tomin1/patience-deck/blob/cb5af775dc39a1d2c645646de7e8a31fc184f3fe/qml/pages/about/AboutPage.qml#L44

Of course Country Quiz doesn't have to do it exactly the same way but that is an option. The one thing I don't like is that this pulls git as a dependency of the package but I don't really see a way around that if the version number should include git tag or the commit if there is no matching tag.