Align XML_Document and XML_Element APIs more.

[jdunkerley]

Pull Request Description

• Added to_table extensions on some core types.
• Added ICON to Any.to.
• Added ICON to Column.info, Table.info, DB_Column.info and DB_Table.info.
• Added defaults to Table.cross_tab and DB_Table.cross_tab.
• Added name, get, at, inner_xml and outer_xml to XML_Document.
• Added constants into left hand side of simple expressions.
• Added widget to get and at on XML_Document and XML_Element. (Some bug in annotation code with Dmitry)
• Altered get and at to not allow XPath and just get direct child/attribute values.
• Added get_xpath to XML_Document.
• Renamed get_elements_by_tag_name to get_descendants_by_tag_name and added new get_children_by_tag_name.
• Added child_names and attribute_names to XML_Document and XML_Element.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.

[radeusgd]

I have a hard time understanding what is the difference between being able to access the class from instantiate it.

Do we need the distinction? Couldn't we throw the same kind of exception for these cases?

Is this exception even expected at all? Isn't it some kind of internal error if it occurs?