config: remove is_attribute

config: remove is_attribute

Simplify code, by removing the ability to differentiate between
attributes and sub-elements when creating node names. For example,
the following two examples would generate the nodename `bar.foo`

- <bar><foo></foo></bar>
- <bar foo="">

In theory, there could be clashes, but I think in reality it is unlikely.
There are no clashes in openbox-spec and it would be pretty confusing to
have something like:

<font name="">
  <name></name>
</font>