Fix spelling errors in code

We could run codespell on the code base to catch some typos. I tried running something like: codespell -S "build,po,*.jpg,*.png,*.svg,*.po,*.gmo,*.o,*.so*,*.gresource,.git,*.flatpak-builder,*.typelib,*.appdata.xml,*.appdata.xml.in,*.desktop,osm-types.json,xmldom,togeojson,geojson-vt,NEWS"

I deliberately skipped NEWS and the appdata.xml.in since they contain release information, and there are typos, but I don't think we should probably rewrite the history of already released versions.