Cleanup: remove our ChooseCaretX in SetSelection. We only use SetSelection/SCI_SETSEL for line comment toggling because it's what SciTE does but it doesn't do ChooseCaretX.
TBH I would rather remove this entire function and only keep SetSingleSelection/SCI_SETSELECTION. This is because SCI_SETSEL is one of the functions predating Scintilla's multiple selection support.