Archetype: add support for conditional blocks

[romain-grecourt]

Add a conditional block to group blocks that are nested with the nested condition.
This will help to avoid repeating the same condition on consecutive blocks.

[tvallin]

If I understand the enhancement correctly, today it works this way:

<output>
      <model>
          <list key="foo" if="bar">
              <value>foo</value>
          </list>
          <list key="foo1" if="bar">
              <value>foo1</value>
          </list>
      </model>
</output>

and we want to do this:

<output>
      <model>
          <condition if="bar">
              <list key="foo">
                  <value>foo</value>
              </list>
              <list key="foo1">
                  <value>foo1</value>
              </list>
           </condition>
      </model>
</output>

[romain-grecourt]

Yes. It would be nicer though if we were able to have <if expr="true"> and <else>.

[tvallin]

Okay, so I can think of two options:

<output>
      <model>
          <if expr="bar">
              <list key="foo">
                  <value>foo</value>
              </list>
              <else/>
              <list key="foo1">
                  <value>foo1</value>
              </list>
           </if>
      </model>
</output>

or

<output>
      <model>
          <if expr="bar">
              <list key="foo">
                  <value>foo</value>
              </list>
          </if>
          <else>
              <list key="foo1">
                  <value>foo1</value>
              </list>
           </else>
      </model>
</output>

[romain-grecourt]

The 2nd one is better, though the XML reader will be a bit tricky.
We should also support else-if