Fix various typos [skip ci]

Found via `codespell v2.1.dev0`  
```
codespell -q 3 -L aci,ake,aline,alle,alledges,alocation,als,ang,anid,apoints,ba,beginn,behaviour,bloaded,byteorder,calculater,cancelled,cancelling,cas,cascade,centimetre,childs,colour,colours,commen,connexion,currenty,dof,doubleclick,dum,eiter,elemente,ende,feld,finde,findf,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/MergeDocuments.cpp

@@ -68,7 +68,7 @@ protected:
     // It is not safe to change potential object name reference at this level.
     // For example, a LinkSub with sub element name Face1 may also be some
     // object's name that may potentially be mapped. In addition, with the
-    // introduction of full quanlified SubName reference, the Sub value inside
+    // introduction of full qualified SubName reference, the Sub value inside
     // LinkSub may require customized mapping. So we move the mapping logic to
     // various link property's Restore() function.
 #if 0