Use glib::Closure for signal connector functions

[sdroege]

No need to use unsafe trampolines anymore.

[sdroege]

I implemented something like this here manually for a signal: sdroege/gstreamer-rs@b58f4cd

The cast for the first argument could be improved to not require all the runtime checks, otherwise we just need to change gir to emit code like this

[sdroege]

Any comments on this, should we generate code like this (or how should it be different?) instead of all the unsafe trampoline magic?

[sdroege]

@EPashkin What do you think? Would you be interested in working on this, it seems like all the trampoline code is yours so it's probably fastest if you look into this. I expect a lot of code (also generally in the code generator) to be possible to remove after this, it could be a major cleanup of gir.

[EPashkin]

@sdroege It interesting, but I prefer postpone this until next release.
Not sure that result code will be better as instead unsafe trampoline there be unsafe parameter casting,
same for gir

[GuillaumeGomez]

After a discussion with @sdroege, he gave me a few extra information that might be useful to solve this issue:

We want to use connect_local() for non-Send/Sync objects and connect() otherwise, and apart from that it's GValue transformations.

We also need to add some more GValue API to have some kind of borrowing get() function.