Fix "request: restore undo/redo buttons missing in KF5 build"

umbrello/uml.cpp
- In constructor, for symmetry with other members pre-initialize member
  m_undoEnabled.
- In function readOptions :
  - Factor toolBar(QLatin1String("mainToolBar")) into local mainToolBar
    pointer to KToolBar.
  - If QT_VERSION major is 5 or larger then
    - add seperator in mainToolBar;
    - set "edit-undo" / "edit-redo" in local QStrings undoIconName and
      redoIconName;
    - set local QIcons undoIcon and redoIcon using QIcon::fromTheme()
      with undoIconName and redoIconName;
    - set undoIcon in editUndo and set redoIcon in editRedo;
    - call mainToolBar->addAction() with argument editUndo and editRedo.

BUG: 449662
FIXED-IN:2.33.80 (KDE releases 22.03.80)