Apply space attribute more generically to multiple structure elements

[proycon]

As found by @kosloot; the following text is currently inconsistent because TEXTDELIMITER for part is empty and explicit leading/trailing spaces will be normalised away. We need a mechanism to override the default TEXTDELIMITER in some instances. Perhaps by generically allowing the "space" attribute (like on <w>).

<FoLiA xmlns="http://ilk.uvt.nl/folia" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="test" version="1.5" >
  <metadata>
      <annotations>
      </annotations>
  </metadata>
  <text xml:id="t1">
    <p xml:id="p1">
      <t>Een test</t>
      <part xml:id="part1">
        <t>Een</t>
      </part>
      <part xml:id="part2">
        <t>test</t>
      </part>
    </p>
  </text>
</FoLiA>

[proycon]

Ok, this was not entirely correct yet. It is now fixed.

• <part> by default has a space as a text delimiter (instead of None), this makes the initial example in this post VALID. So instead of overriding the default textdelimiter for part, we changed the default.
• space="no" may be set on <part> and almost any other structure element to undo (override) their default text delimiter, making it glue directly to the structural predecessor.
• Some structural elements such as <br> and <whitespace> have an empty textdelimiter (note that "" != None) by definition, those can never be made 'spacing' (because that would be non-sensible anyway).