Rework AttributeModifier to deprecate AttributeAppender and SimpleAttributeModifier

[Martijn Dashorst]

Taken from the user@ list, where it might get snowed under...

The SimpleAttributeModifier, AttributeAppender and AttributePrepender
classes are in my opinion stop gap measures to work around issues with
AttributeModifier's API.

I think that we could do better API wise with three factory methods (or 6 if
we duplicate each method to take an IModel<?> and a String for
convenience) on AttributeModifier, and deprecate SimpleAttributeModifier
and AttributeAppender. I'd also merge the appender/prepender logic into
attribute modifier, such that user specializations can make use of
them.

AttributeModifier.setAttribute(String attribute, IModel<?> value);
AttributeModifier.setAttribute(String attribute, String value);
AttributeModifier.prependAttribute(String attribute, String value);
AttributeModifier.prependAttribute(String attribute, IModel<?> value);
AttributeModifier.appendAttribute(String attribute, String value);
AttributeModifier.appendAttribute(String attribute, IModel<?> value);

Or perhaps:

AttributeModifier.overwrite(String attribute, IModel<?> value);
AttributeModifier.overwrite(String attribute, String value);
AttributeModifier.prepend(String attribute, IModel<?> value);
AttributeModifier.prepend(String attribute, String value);
AttributeModifier.append(String attribute, IModel<?> value);
AttributeModifier.append(String attribute, String value);

I'd value input on the naming of things.

Martijn