Replace the int of the SubType of SelectionChanges with an enum class.
The meaning of it is nowhere documented and some magic numbers like 0,1,2 are used in several places in the code.
Replace the int of the 'resolve' argument of several functions with a proper enum class.
* This avoids the inconsistencies in client code where often true/false is passed when an int is expected
* This avoids the use of magic numbers like 0, 1, 2 or the undocumented 3
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`
- in SelectionSingleton::setVisible, if the selection changes during
loop, a crash may occur.
- in QGraphicsScene, hiding an item changes its selected status.
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)