...which held the position of an item in the GraphicsScene of the net oriented sample editor

import edtools as et
import re

def user_edit_pair(fname_stem, th, tc):

    data = []
    mm = re.finditer('const QString (\w+)::([MPT]_[A-Z0-9_]+) = "(.+?)";', tc)
    for m in mm:
        data.append([m.group(1), m.group(2), m.group(3)])
    mm = re.finditer('const QString (\w+)::([MPT]_[A-Z0-9_]+)\("(.+?)"\);', tc)
    for m in mm:
        data.append([m.group(1), m.group(2), m.group(3)])

    for cname, typ, label in data:
        rlabel = re.sub(r'\[', r'\\[', label)
        rlabel = re.sub(r'\]', r'\\]', rlabel)
        rlabel = re.sub(r'\(', r'\\(', rlabel)
        rlabel = re.sub(r'\)', r'\\)', rlabel)
        tc = re.sub('\n+const QString '+cname+'::'+typ+' =\s+"'+rlabel+'";', '', tc)
        tc = re.sub('\n+const QString '+cname+'::'+typ+'\("'+rlabel+'"\);', '', tc)
        th = re.sub(r'class( BA_CORE_API_)? ('+cname+r'.+?)static const QString '+typ+r';(.*?\n};)',
                    r'class \2static constexpr auto '+typ+r'{"'+label+r'"};\3', th, 0, re.S)

    return th, tc

et.ed_argfilepairs(user_edit_pair)

instead use ItemWithMaterial. This makes the handling more understandable and as a second effect reduces code complexity

and:
* use concrete types like ItemWithMaterial* instead of SessionItem*
* use QVector instead std::deque + QVector to simplify code
* remove now obsolete code
* adapt unit tests

* in an ItemWithMaterial, store only the identifier of the used material
* hold the identifier as QString, not within an ExternalProperty
* modify the ItemWithMaterial/Material property to type "QString" instead of ExternalProperty
* adapt all methods which used the ExternalProperty to use the material identifier or a ptr to a MaterialItem
* adapt the XML reading/writing
* add legacy code to read old XML files
* adapt material selection via SessionModel's CustomEditors

instead get the name directly from the referred material

instead get the color directly from the referred material

Introduce class ItemWithParticles as base for ParticleItem et. al. Make SessionGraphicsItem virtual base class.