Found via `codespell -q 3 -L aci,ake,aline,alle,alledges,alocation,als,ang,anid,anormal,apoints,ba,beginn,behaviour,bloaded,bottome,byteorder,calculater,cancelled,cancelling,cas,cascade,centimetre,childrens,childs,colour,colours,commen,connexion,currenty,dof,doubleclick,dum,eiter,elemente,ende,feld,finde,findf,freez,hist,iff,indicies,initialisation,initialise,initialised,initialises,initialisiert,inout,ist,itsel,kilometre,lod,mantatory,methode,metres,millimetre,modell,nd,noe,normale,normaly,nto,numer,oce,oder,ontop,orgin,orginx,orginy,ot,pard,parm,parms,pres,programm,que,rady,recurrance,ro,rougly,seperator,serie,sinc,siz,strack,substraction,te,technic,thist,thru,tread,uint,unter,uptodate,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/Base/StackWalker.cpp,./build/doc/SourceDocu`
With this, add/remove buttons for displacement constraint will put the user in a
selection mode if pressed when nothing is selected.
Superclass `TaskFemConstraintOnBoundary` Allows code reuse for other
constraints. We do not need this in `TaskFemConstraints` because it is only used
in a subset of constraints, particularly that applies on only some of the
boundary entities.
TODO: Confirm Undo/Redo works appropriately.
TODO: Optimize `onSelectionChanged()`.
