[FR] Allow daisy-chaining when adding children to ValueTrees

Feature Requests
2

This would go nicely with [FR] Default UndoManager arguments to nullptr in all ValueTree methods - #6 by ImJimmi to make constructing value trees this way even cleaner:

juce::ValueTree {"Root"}
    .setProperty("foo", 10)
    .setProperty("bar", 20)
    .appendChild (juce::ValueTree {"Branch"}
                      .setProperty ("x", y)
                      .appendChild (juce::ValueTree {"Leaf"}))
    .addChild (juce::ValueTree {"Branch"}, 0);