[Accessibility API] add alternate text property.

Right now the only way to set a text to an ImageView for the screenreader is to subclass it, overwrite accGetAccessible() and handle TITLE. It should not be that hard. Although RT-37749 can make that a bit easier it is still not easy enough.

We initially considered to add an alternateText property to ImageView and Canvas.

It can also be useful for Labeled to have such a property in order to allow the user to easily define a text content for the screen reader different (more descriptive) than Labeled#text.

Following that line of thought maybe alternateText() could be add to Node.

Possible names for this new property are: name, altText, alternateText, alternativeText.

We can also simplify the setting of other commonly used attributes by adding properties, they are:
help, description, role

Therefore, a complete solution solution for example, is adding to Node nameProperty(), helpProperty(), roleProperty() and descriptionProperty().

The minimal recommented solution is adding alternateText to ImageView and Canvas.