Update tutorials to imagej-2.0.0-beta-7.7

The main difference is the change of CommandService#run method
signatures to include a required "boolean process" flag.
HelloWorld: fix bug where nothing appears

In order for inputs to be harvested and outputs to be displayed,
there must be a currently visible UI. Otherwise, nothing happens.
ExecuteCommands: use OpenFile.class signature

This is more type-safe than passing the class name as a string. Really,
we want to highly discourage use of the "class name as string" signature
since it can result in buggy dependency structures.
ExecuteCommands: add a 4th approach: service calls

I think service calls are actually the best approach for structuring
API, and hence for calling the functionality of commands.
WorkingWithModules: bring explanation up to date

It is no longer the case that ModuleService never does processing while
CommandService always does. That was too inflexible. Now we have a
"boolean process" flag for all run signatures at both levels, so it is
explicitly optional whether pre- and postprocessing occur.