Fix various (doxy) typos and whitespace issues

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`

src/App/PropertyContainer.cpp

@@ -244,9 +244,9 @@ void PropertyContainer::Save (Base::Writer &writer) const
     writer.Stream() << writer.ind() << "<Properties Count=\"" << Map.size()
                     << "\" TransientCount=\"" << transients.size() << "\">" << endl;
 
-    // First store transient properties to persisit their status value. We use
+    // First store transient properties to persist their status value. We use
     // a new element named "_Property" so that the save file can be opened by
-    // older version FC.
+    // older versions of FC.
     writer.incInd();
     for(auto prop : transients) {
         writer.Stream() << writer.ind() << "<_Property name=\"" << prop->getName() 
@@ -635,4 +635,3 @@ void PropertyContainer::Restore(Base::Reader &reader)
 \endcode
 
 */
-