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/Gui/Document.cpp

@@ -991,7 +991,7 @@ bool Document::save(void)
             if(docs.size()>1) {
                 int ret = QMessageBox::question(getMainWindow(),
                         QObject::tr("Save dependent files"),
-                        QObject::tr("The file contain external depencencies. "
+                        QObject::tr("The file contains external dependencies. "
                         "Do you want to save the dependent files, too?"),
                         QMessageBox::Yes,QMessageBox::No);
                 if (ret != QMessageBox::Yes) {
@@ -2239,4 +2239,3 @@ void Document::slotChangePropertyEditor(const App::Document &doc, const App::Pro
         setModified(true);
     }
 }
-