Add Execute() D-Bus method to org.xfce.FileManager.

This function takes a working directory, a URI or relative or absolute
path of a file to execute, an array of URIs, relative or absolute paths
of files to supply to the executed file, as well as a display name and a
startup ID.

In its essence, it's a simple wrapper around thunar_file_execute() so
this method can be re-used by other processes.