Derived from App::DocumentObjectT to add support of sub object
Also some minor changes to App::DocumentObjectT
Changed Gui::SelectionChanges to make use of SubObjectT
Found via `codespell -q 3 -L aci,ake,aline,alle,alledges,alocation,als,ang,anid,ba,beginn,behaviour,bloaded,byteorder,calculater,cancelled,cancelling,cas,cascade,centimetre,childs,colour,colours,commen,currenty,dof,doubleclick,dum,eiter,elemente,feld,freez,hist,iff,indicies,initialisation,initialise,initialised,initialises,initialisiert,ist,kilometre,lod,mantatory,methode,metres,millimetre,modell,nd,noe,normale,normaly,nto,numer,oder,orgin,orginx,orginy,ot,pard,pres,programm,que,recurrance,rougly,seperator,serie,sinc,strack,substraction,te,thist,thru,tread,uint,unter,vertexes,wallthickness,whitespaces -S ./.git,*.po,*.ts,./ChangeLog.txt,./src/3rdParty,./src/Mod/Assembly/App/opendcm,./src/CXX,./src/zipios++,./src/Base/swig*,./src/Mod/Robot/App/kdl_cp,./src/Mod/Import/App/SCL,./src/WindowsInstaller,./src/Doc/FreeCAD.uml`
Mostly about API changes to SelectionSingleton. Also include related
changes to SelectionObject, SelectionChange, and SelectionView.
Please check out the summary of changes [here](https://git.io/fjimC)
Selection callback are sometimes called while traversing Coin node tree.
If any unhandled exception is thrown while traversing the tree, FC 3D
rendering will become unstable and non-usable thereafter.
