Pass string literals directly to execute_command()

There were a few cases of creating a local string variable from a
literal and then passing the variable to execute_command() like this:
    Glib::ustring cmd = "whatever";
    Utils::execute_command( cmd, ... );

This creates an unnecessary local variable.  Instead pass the string
literal directly to Utils::execute_command() like this:
    Utils::execute_command( "whatever", ... );
This also make the code a little bit more grep friendly.