Add default value capability at stereotype attribute:

umbrello/uml1model/stereotype.h
- In class AttributeDef,
  - add member defaultVal of type QString;
  - extend constructor for externally provided defaultVal.

umbrello/uml1model/stereotype.cpp
- In function saveToXMI1 loop over m_attrDefs, if ad.defaultVal is
  non empty then setAttribute "initialValue" on attElement.
- In function load1 traversal of QDomNodes tag "UML:Stereotype.feature"
  while-loop over attElem tag "UML:Attribute" retrieve attElem attribute
  "initialValue" into local dfltVal of type QString.
  Pass dfltVal into AttributeDef constructor.

umbrello/dialogs/stereoattributedialog.h
- Add member m_pDefaultValueLabel[N_STEREOATTRS] of type QLabel*.
- Add member m_pDefaultValueEdit [N_STEREOATTRS] of type KLineEdit*.

umbrello/dialogs/stereoattributedialog.cpp
- In function setupDialog() for-loop of row,
  - place m_pDefaultValueLabel[row], m_pDefaultValueEdit[row] into
    columns 4, 5 using Dialog_Utils::makeLabeledEditField();
  - in case (adefs.count() > row) initialize m_pDefaultValueEdit[row]
    from adefs.at(row).defaultVal.
- In function apply() pass m_pDefaultValueEdit[i]->text().trimmed()
  into UMLStereotype::AttributeDef constructor argument dfltVal.

CCBUG: 73847