snowap: Add a weak version of the signaler

[Ph4ntomas]

While porting Glacier's widgets to composable programs, I noticed there was no weak version of the Signaler.

My goal was to create a simple handle for some widget that would use the Signaler to emit messages targeted to the widget, but keeping the Signaler around is leaky.

The alternative are:
a. Make the widget state an Arc and the handle a Weak of that state
b. Pass the surface handle to subsystems that need to send message
c. Make the widget listen to some events and drive them via an external Signaler

I'd rather avoid (a) whenever possible, and (b) give more capabilities than strictly required.
(c) does works, but I'd prefer having the option to do both.

EDIT:
Regarding (c): while it does work, it's made easier to use with the PR since the signal handler can capture a WeakSignaler. The alternative implementation is to setup the signal after the surface Created event, so we can capture the surface handle instead, which is less than ideal.

[Ph4ntomas]

I haven't check whether the lua side need something similar, I'm currently porting the rust code first. If needed, this MR can wait until after I'm done, otherwise I'll open another one for lua if I find the need to.

[Ottatop]

Thanks!