Math commands used in text mode now always use the current selection as contents

Math commands used in text mode now always use the current selection as contents
(#9762)

* fixes a bug where this was already the expected behaviour of
  math-subscript and math-superscript but failed.

* corrects the behaviour where if there is \newcommand in the
  selection, then a corresponding macro template is introduced
  instead of a math inset.

  * fixes a bug where math-display, math-subscript and math-supscript
    would also introduce such a macro template in a way unrelated to
    their function. Now it only happens with math-mode without
    arguments.

  * fixes a bug where a text that does not denote a macro definition,
    e.g. "aaa\newcommandaaa", would produce \invalidmacro.