Provides support for reusing certain property specifications that are likely to be the same, given how images in a GChart get reused, and given certain assumptions about which properties of the image are most likely to remain unchanged between updates. For the most common scenarios, chart updates are significantly faster due to replacing (relatively expensive) DOM style attribute setting with (relatively cheap) String reference or integer equality tests.
For hovertext, the class also lets us defer actual generation of the hovertext until they actually mouse over the image, saving further time (it's surprisingly expensive just to format the numbers and such used in hovertexts).
TODO: Since we no longer use events on Image widgets, see if we can switch to just using simpler HTML elements, if that reduces the overhead associated with a Widget?