Enable XML, and DataEntry should allow setting XML attributes

[A248]

• XML is the only backend format I'm aware of that distinguishes between elements and attributes. We can support setting attributes by adding a flag on DataEntry, along with an annotation @Attribute
• Investigate how to use StAX API with Woodstox.
• Add an XML backend.
  • Non-string keys have to be rejected, I believe.
  • Lists can be implemented in XML files the same way they are in javadoc, by using <li> tags under the section.

As a reminder, Backend implementations should throw DeveloperMistakeException if the format does not support the data structure written to it. This means that attempting to ram a list or a map into an XML attribute will be rejected when the configuration is written. See javadoc of Backend#write

[A248]

Here's a better idea to distinguish between elements and attributes. We can incorporate a concept that exists across configuration formats, not just XML.

Entries can hint whether they should be stored "inline" or in "expanded" form. Expanded is the default and corresponds to the tree-like layout exemplified by block YAML. Inline is what we might consider an attribute; it is something that typically exists on the same line, in a compact form.

Lists and maps, depending on the backend, may not be able to be stored inline. XML is an example of this. Given a tree of values (DataTree), key-value pairs can be stored inline if the value is a scaler. Hence, something like this works:

<mytree val1="hi" val2 = "bye">

It corresponds to the following data tree, where the preferInline method establishes that the value should be inlined if possible:

DataTree.Mut tree = new DataTree.Mut();
tree.put("val1", new DataEntry("hi").preferInline(true));
tree.put("val2", new DataEntry("bye").preferInline(true));