Address observation from https://bugs.kde.org/show_bug.cgi?id=447866#c2

> [...]
> Another thing I noticed is that if I save a file, it does not add the
> ".xmi" extension if I do not type it explicitly.

umbrello/uml.cpp function slotFileSaveAs
- In loop, after call to QFileDialog::getSaveFileUrl() :
  - Use `break' instead of setting `cont' false to avoid unnecessary
    if-nesting depth.
  - Before testing QFile::exists(file), if `file' does not contain "."
    then append extension ".xmi".
  - On calling KMessageBox::warningContinueCancel() use `file' in lieu
    of url.toLocalFile().

CCBUG: 447866