Inspection never shows children in a tree, causes errors for other node types

[JanBliznicenko]

There are multiple issues caused by b55512b

The condition for isContentNode in XMLNode>>#inspectTreeIn: is reversed, so something that has descendantElements (which is only XMLNodeWithElements) always gets no children in the tree, making the tree always have only 1 level. Inversely, anything else, that cannot have any descendants, like XMLAttribute, tries to get these descendantElements, still resulting in an error that the commit attempted to fix.

The fix only works for XMLString, as it returns true for isContentNode, so it does not try to return children as well.

I believe one of the following can be done:

Option 1:

• remove isContentNode from XMLString, therefore not considering it a content node anymore (which might not be a correct thing to do, don't know the intended semantics for what is considered a "content node" and reasons why this method exists)
• swap contents of ifTrue: and ifFalse: blocks in XMLNode>>#inspectTreeIn:

Option 2:

• introduce another typecheck-like testing method, named for example "canHaveDescendantElements", that would return false for XMLNode, true for XMLNodeWithElements (therefore XMLString would also return false)
• use this canHaveDescendantElements instead of isContentNode in the condition in XMLNode>>#inspectTreeIn:
• swap contents of ifTrue: and ifFalse: blocks in XMLNode>>#inspectTreeIn:

Option 3:

• Create method descendantElements in XMLNode that would return an empty array
• remove the condition in XMLNode>>#inspectTreeIn: altogether and always get descendantElements