https://bugs.kde.org/show_bug.cgi?id=347498#c8

> I experienced the following crash on exiting Umbrello:
> Application: Umbrello...  (umbrello5), signal: Segmentation fault
> [...]
> AFAICT the critical place is umlobject.cpp ... setUMLStereotype,
>    emitModified();
> We should not emit the signal during destruction.

umbrello/uml.{h,cpp}
- Add public static function shuttingDown() returning bool.
- Add private static variable s_shuttingDown with documentation on why
  this variable is static.
- In UMLApp destructor set s_shuttingDown to true.

- In function setUMLStereotype(UMLStereotype) conditionalize call to
  emitModified() on UMLApp::shuttingDown() returning false.

CCBUG:347498