Fix for "XMI load in git version fails"

On loading the file, after a few seconds Umbrello freezes the entire
system and `top` indicates in excess of 19GB VIRT and several gigabytes
RES - presumably swapping ensues.

I was able to trace it to UMLDoc::loadFromXMI (umldoc.cpp:2478)

  emit sigWriteToStatusBar(i18n("Setting up the document..."));
  qApp->processEvents();  // give UI events a chance    <--- line 2478
  activateAllViews();

and apparently there are very many events queued. Some seconds later the
freeze occurs.  On one repetition I was able to press <Ctrl-C> in gdb
just before the freeze, and that's where the bad `rect` parameter of
UMLScene::drawBackground showed up:

#4  0x000000000073ba06 in UMLScene::drawBackground (this=.., painter=..,
                 rect=...) at /tools/umbrello/umbrello/umlscene.cpp:3760
(gdb) p rect
$2 = (const QRectF &) @0x7fffffffa3d0: {xp = -4160651296.6437497,
                                        yp = -18968990857.816666,
                                        w = 4026436738.6874995,
                                        h = 33643115861.033333}

Code change:
umbrello/umlscene.cpp
- Define static const qreal maxCanvasSize with value 100000.0
  (subject to discussion - could be enlarged).
- In function drawBackground reject the request if rect's x or y
  is outside the range -maxCanvasSize to maxCanvasSize, or if rect's
  width or height is less than 0 or greater than maxCanvasSize.

CCBUG:449393