Add size tracking to preview window title

[gluon-bot]

Originally reported by: Luca Domenichini (Bitbucket: mimmoz81, GitHub: mimmoz81)

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It would be useful to have the title of the preview window (Ctrl + P) to report the size x of the preview window.
This way, one can better see how the scene will render at specific resolution..

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• Bitbucket: https://bitbucket.org/gluon-oss/scenebuilder/issue/59

[AlmasB]

Implementation is effectively this or a variation of. Note that this tracks the stage size, rather than scene size.

In PreviewWindowController:

getStage().titleProperty().bind(
                new SimpleStringProperty(Utils.makeTitle(fxomDocument))
                        .concat(getStage().widthProperty().asString(" (%.1f x "))
                        .concat(getStage().heightProperty().asString("%.1f)"))
        );

Just need to confirm this feature is worth implementing.

[Oliver-Loeffler]

This is a small but very helpful function. I would welcome its implementation.