Add error messages for incorrectly nested XML3D elements

[lachsen]

Especially when converting XML3D tag code from XHTML to HTML it often happens, that tags are not "correctly" closed in the HTML version.

For example, this HTML code:

<float3 name="position" param="true" />
<float3 name="normal" param="true" />
<int4 name="boneIdx" param="true" />
<float4 name="boneWeight" param="true" />

will be parsed to this DOM representation:

<float3 name="position" param="true" >
    <float3 name="normal" param="true" >
        <int4 name="boneIdx" param="true" >
            <float4 name="boneWeight" param="true" ></float4>
        </int4>
    </float3>
</float3>

Those mistakes are difficult to detect because there is no error message about this.

Thus, I suggest to add a couple of additional checks for the node hierarchy that makes it easier to detect those mistakes.

An example: when nesting value element within each other (e.g. an <int> within a <float3>) an error or warning should appear.