https://bugs.kde.org/show_bug.cgi?id=449393#c5 followup to commit 6085771b

> "Fixed" just means that checks were added to refuse to set rogue x/y
> coordinate value on widgets.

umbrello/umlscene.{h,cpp}
- Change the impl local static const maxCanvasSize to class level static
  s_maxCanvasSize.
- Add getter maxCanvasSize() returning s_maxCanvasSize.

umbrello/umlwidgets/umlwidget.cpp
- In functions setX and setY, if the argument is below -maxCanvasSize()
  or above maxCanvasSize() then log an error and do not change the
  widget coordinate.

CCBUG: 449393