Var user data parameter does not work for connect() makro

[StefanSalewski]

Extending the existing mconnect() makro in gimpl.nim does not work currently due to

https://github.com/nim-lang/Nim/issues/14605

So we can not really examine the user provided function to see if a var parameter is used.

What we could do is provide a macro with another name like varConnect() but that seems to be not very nice, as we have already connectAfter() and connectSwapped(). And var parameters are needed in few cases only, tests/simple of GTK4 may be a use case. But we can just use a ref or a global variable for now.