Implement properly \limits and \nolimits

Implement properly \limits and \nolimits

These are now properties of insets that can be operators :
InsetMathSymbols, InsetMathDecoration (for over/underbrace) and
InsetMathMacro (for its contents).

Each of these has a limit_ member that allows to remember a limit
forcing and a member defaultLimits() that indicates what to do in the
absence of such forcing. Moreover the write() method calls
writeLimits().

This allows to simplify the definitions of integrals in lib/symbols by
defining the integrals as macros of their "op" version, as it is done in
the style files.

Also, many hardcoded assumptions can now be removed.

The handling of LFUN_MATH_LIMITS is now done in InsetNest, which tries
successively to apply the limit change to (1) the character after
cursor, (2) the character before cursor and (3) the character at the
end of the inset (useful for script insets?)

The new code allows to define
  \newcommand\int{\intop\limits}
but not
  \newcommand\makelimits#1{#1\limits}

It is also possible to type explicitly \limits or \nolimits to modify
a symbol.