Get rid of the symbol import hack in qstdweb

It is impossible to add a literal $  on current cmake version (see https://stackoverflow.com/questions/68775616/how-not-to-have-dollar-sign-in-target-link-options-mangled).

A dollar sign would be needed if we wanted to include a JS library emscripten function (we need JSEvents and specialHTMLTargets).

To get the symbols exported, we use a visible symbol 
emscripten_set_wheel_callback in qstdweb.cpp. The symbol depends on $JSEvents and $specialHTMLTargets, so these two get included.
Ideally we would like to 
emcc -s EXPORTED_RUNTIME_METHODS=JSEvents,specialHTMLTargets DEFAULT_LIBRARY_FUNCS_TO_INCLUDE=$JSEvents,$specialHTMLTargets
but this is currently impossible. Revise this when cmake gets fixed