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.