Renamed BoxPlug min() and max() methods to minPlug() and maxPlug().

This keeps them in line with all the other plug accessors elsewhere.
Improved CompoundDataPlugValueWidget alignment for box plugs.

The plug label now appears at the top rather than floating in the middle. Applying the fixed size really isn't a great idea, but I've spent all morning seeking alternatives without luck. The best attempt was to set the QLabel stylesheet to have border=0px, padding=1px and margin=0px, which although different to the QLineEdit stylesheet, would give an equivalent height for standard (no HTML styles applied) text. Unfortunately that caused the layout to jump when the summary text in the collapsible headers for StandardOptions nodes went from "" to "something".