property_tree/xml_parser is conceptually flawed, because it can't parse XMLs with the mix of texts and child trees

This XML:
```

Text1
a
Text2
b
Text3

```
parse result has all texts (Text1, Text2, Text3) combined into a single string that is available as the output of the ```data()``` function:
```
data="
Text1

Text2

Text3
"
```
and boost leaves no way to determine that `````` and `````` are in the middle of this text. ```tag1``` and ```tag2``` are available as separate keys, but this loses information about their location.

A larger practical example:
The line ```const GLfloat *v``` in https://raw.githubusercontent.com/KhronosGroup/OpenGL-Registry/master/xml/gl.xml can't be parsed using ```property_tree/xml_parser``` because the `````` tag is between two text elements.

Start by reproducing the XML example from the issue and inspect the property_tree XML parser behavior. Compare the result with the mixed-content structure in the sample and the param element in KhronosGroup/OpenGL-Registry's gl.xml. Done requires a decided representation that preserves the ordering of text and child elements, along with coverage for the reported cases.

Written by the indexing model from the issue text.