FYI this feature is also required by #81364.

Umbrello doesn't even have subsystems yet and I see that depending
on the UML version there are different ways of presenting a subsystem.
For simplicity I think I'll opt for the UML 1.5 style presentation
using a package with a stereotype <<subsystem>> (at least as a first
implementation, maybe we can extend that later.)

SVN commit 528227 by okellogg:

Make components nestable.
Up next: Do the same for subsystems.
CCBUG:124326


 M  +2 -0      assocrules.cpp  
 M  +48 -8     component.cpp  
 M  +5 -8      component.h  
 M  +43 -32    listpopupmenu.cpp  
 M  +7 -4      model_utils.cpp  
 M  +53 -10    umllistview.cpp  
 M  +3 -3      umllistview.h  
 M  +59 -45    umlview.cpp  
 M  +1 -1      umlviewimageexportermodel.cpp  

Awesome!  Thanks!

SVN commit 528933 by okellogg:

Implement subsystems for component diagrams.
A subsystem is internally represented by a package with the stereotype <<subsystem>>.
This probably still has some rough egdes. Please test and leave your comments at
BUG:124326


 M  +1 -0      assocrules.cpp  
 M  +27 -1     listpopupmenu.cpp  
 M  +2 -0      listpopupmenu.h  
 M  +11 -5     packagewidget.cpp  
 M  +1 -0      pics/Makefile.am  
 A             pics/sources/subsystem.svg  
 AM            pics/subsystem.png  
 M  +72 -10    umllistview.cpp  
 M  +3 -1      umllistview.h  
 M  +5 -1      umllistviewitem.cpp  
 M  +1 -0      umlnamespace.h  
 M  +10 -6     umlview.cpp  

I'd love to try it out, but I can't get code from SVN to work.  I get weird
automake/unsermake/etc. errors.  I'll just wait for the release, I guess.