Don't create the welcome if a layout was passed

[mgotz]

Address #2159 by not creating a welcome tab, if the -layout was passed.

[kasemir]

As you say in the last commit, "order matters".
I'm a little worried about the way -layout /path/to/xxx.memento is only partially removed, and then there's this check

         if (!application_parameters.contains("-clean") &&
             !restoreState(memento) &&
             !application_parameters.remove("-layout"))

which will remove the remaining -layout from the application_parameters, but only if there's no -clean flag, because otherwise the shortcut-behavior of && will skip the application_parameters.remove(.

Please triple-check that all command line combinations still "work", and/or simplify by introducing some new "suppress_welcome" flag that's set when first handling the -layout, then remove the complete -layout at that time.

[kasemir]

Alternate PR uses an explicit show_welcome flag.

[kasemir]

Handled by #2184