Set CommandResult, Exception & save EntryBook independently of Command

[rlrh]

Modify design to enable getting and setting of CommandResult and Exception, and EntryBook auto-saving, independently of any Commands, with UI updating accordingly

Resolves #34

Supports future commands that need to return an initial result first, do some long-running work, and then return another final result or exception, and also internal tasks that modify the model and need to return a user-facing result or exception

• Decouples saving address book from command execution - makes address book save itself to storage whenever it is modified, even if not through command execution
• Makes exception observable and enables it to be manually set (currently used for data-saving exception only)
• Makes manually set command result observable and enables it to be manually set (currently unused)
• Makes UI observe command result and exception and update appropriately, in addition to currently updating on command execution
• Modifies existing tests to support new implementation, adds tests for new capabilities

New:

A command can now return an initial command result first, do some long-running task that involves modifying the model, which will save itself to storage, and then update the observable command result or exception, which can then be handled appropriately by the UI.

Old:

A command can only return an initial command result, and then do some long-running task that involves modifying the model, but it will not save itself to storage, and a final command result or exception cannot be propagated.

How to Use:

In a Command's execute method, return an initial CommandResult, and within an asynchronous task, set the Model's commandResultProperty or exceptionProperty.
Example:

Task<Void> task = new Task<>() {
    @Override
    public Void call() throws Exception {
        try {
            Thread.sleep(5000); // some long-running work
            Platform.runLater(() -> {
                model.setCommandResult(new CommandResult(MESSAGE_SUCCESS));
            });
        } catch (Exception e) {
            Platform.runLater(() -> {
                model.setException(new Exception(MESSAGE_FAILURE));
            });
        } finally {
            return null;
        }
    }
};
new Thread(task).start();
return new CommandResult(String.format(MESSAGE_IN_PROGRESS, toAdd));

[epicfailname]

Note to selves that this will be the second time this function is called. The first time is in the line above, within the MainWindow's executeCommand.