Improve handling of binops on LHS expressions

Improve handling of binops on LHS expressions

Add a generic method to handle binops such as +=
to Object and use it. We can probably remove
most of the __qmljs_inplace_xxx methods later
on and call one generic method instead.

Change-Id: If7cbd2e3012ad381e29b9eec2059e17f02a0a38a
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>