Dom: use URL's instead of paths

The Dom uses filePaths everywhere, but the LSP standard only works with URL's. Instead of always converting the filepaths to URL's (which might lead to some problems later on), change the Dom to always use URL's.

Also, this avoids mishaps between filePaths and URL's: Some language clients accept filePaths (e.g, VS Code) but other show an error (e.g., Kate) when qmlls returns a filePath instead of a URL.