Address https://bugs.kde.org/show_bug.cgi?id=449622#c4 ,

> I looked into this and there is one line in the file that does not fit
> with this approach.  It is line 1895,
> <floatingtext [...] x="-4.29496e+09"  y="-4.295e+09" text="sprites"
> In order for the approach to work, this line needs to be removed
> manually.

Improve widget coordinate error handling so that no manual intervention
is needed:

umbrello/umlscene.{h,cpp}
- Make function maxCanvasSize() static and document it.

umbrello/umlwidgets/umlwidget.cpp
- In functions setX and setY call UMLScene::maxCanvasSize() as a static
  function.
- In function loadFromXMI, if nX or nY are outside the range
    -UMLScene::maxCanvasSize() .. UMLScene::maxCanvasSize()
  then log a warning, set them to a fixed small value, and do not
  apply offset correction.

CCBUG: 449622