public BooleanProperty keepAspectProperty() {
        if (null == keepAspect) { keepAspect = new SimpleBooleanProperty(Tile.this, "keepAspect", _keepAspect); }
        return keepAspect;
    }
Defines if the control should keep it's aspect. This is
in principle only needed if the control has different width and
height.

@param KEEP
src/main/java/eu/hansolo/tilesfx/Tile.java