Fixes to input method handling

Fixes to input method handling

The goal of this patch is to make "self-insert <string>" act as a
sequence of individual self-insert calls. Once it is done, the
insertion of the commit string in GuiWorkArea can use that, which
fixes the issues described in bug 11183.

1/ in InsetMathNest, SELF_INSERT is rewritten to be a proper loop of
   interpretChar, instead of inserting the string as characters. This
   solves the issue of interpreting ^ properly.

2/ the text version does not need to be changed (the loop is rewritten
   using a range, though).

3/ Then {{{GuiWorkArea::inputMethodEvent}}} is changed to invoke
   directly self-insert. This removes the update issues.

Fixes bug 11183.

(cherry picked from commit ad5548cfe39a17389ca721aab896e876f2cd4124)