First attempt to update code for GTK4

[smartyg]

First attempt to update osm-gps-map to GTK4. So far I only updated the main code. I will try to update the examples in the coming days. Please note this branch is not finished/final yet, it is still work-in-progress. Also note this branch was made on top of my own master (which for example also includes #95). Once I feel comfortable enough, I will do a rebase of the GTK4 update based on the nzjrs/osm-gps-map/master branch.

Please feel free to test, review, suggest changes or give any other comments.

I hope any one is interested in this update.