Fix math mode for InsetMathMacro on output

Fix math mode for InsetMathMacro on output

After 6642152e, user macros were no longer wrapped in \ensuremath. In 2.2 and
before, InsetMathMacro behaved as follow:

* Textmode global symbols are wrapped in \text when in math.

* Other global symbols, and user macros, are wrapped in \ensuremath when in
  text.

* Undefined macros (ERT) are wrapped neither in \text nor in \ensuremath.

This is also consistent with the documentation of MathEnsurer in
mathed/MathStream.h.

This patch defines InsetMathMacro::currentMode() accordingly (respectively
TEXT_MODE, MATH_MODE and UNDECIDED_MODE) and uses it to determine the output.

After this patch, there is a mismatch between screen and pdf output for user
macros in \text. This is not a regression wrt 2.2 and is because linearization
does not satisfy currentMode() currently.