LocationChooser improvements

It's somewhat easy to scroll or pan away from the pin you placed, so let's add an action to re-center the map if you want to find it again.

The map centers on London by default, but for the other people living outside it may find it hard to figure out where they are. This adds a button that calls into QtPositioning to center the map over where you are.

Note while testing the "Locate" button may not work, you need to make sure you're not running into https://gitlab.freedesktop.org/geoclue/geoclue/-/issues/186. I'm not sure whether or not this works on Android, we might need to ask for permissions.