Fix the last clang warnings about overloaded virtual methods

This patch fixes a series of warnings like:
{{{
In file included from ../../master/src/mathed/InsetMathBoldSymbol.cpp:13:
In file included from ../../master/src/mathed/InsetMathBoldSymbol.h:15:
../../master/src/mathed/InsetMathNest.h:37:7: warning: 'lyx::InsetMathNest::metrics' hides overloaded virtual function [-Woverloaded-virtual]
        void metrics(MetricsInfo const & mi) const;
             ^
../../master/src/insets/Inset.h:186:15: note: hidden overloaded virtual function 'lyx::Inset::metrics' declared here: different number of parameters
      (2 vs 1)
        virtual void metrics(MetricsInfo & mi, Dimension & dim) const = 0;
                     ^
}}}
For a description of the problem, see for example:
http://stackoverflow.com/questions/18515183/c-overloaded-virtual-function-warning-by-clang

3 different strategies have been used:
 * in frontend, some functions have been renamed.
 * in InsetMath.h, Inset::write has been explicitly imported too
 * in InsetMathNest.h, since a comment said that hiding Inset::metrics is intended, a special trick has bee used to silence the warning.