Default relative database filename to a sensible location

As it is, the path of a Database can be literally anything. A relative path will put it wherever you happen to be running the application from, which is not very useful. A full path can be given, albeit there's no easy way to know it.

The relative path should by default be something like ~/.local/share/$APP_ID which would conveniently work under confinement and also be the right thing to do under a classic freedesktop.org system.

Full paths will still be allowed as they are and usable on the desktop, as will the special :memory: filename.