Fix bug 'don't generate the code with postgresql option in code generation wizard'.

- Use uml object root depending on the active language instead of tree view
  selection, which do not work for entities.
- Fix order of code generator wizard pages to show options first and then
  class selection page to avoid choosing invalid objects caused by changing
  active language after selection.

BUG:354700
FIXED-IN: 2.17.3 (KDE Applications 15.08.3)