Remove unnecessary `IO`

[Kritzefitz]

This PR removes the unnecessary IO from the return value of signalAddHandler. It isn't needed because the argument to the AddHandler constructor already is IO.

I took the liberty of updating the dependencies in the stack.yaml, because I needed a newer version of haskell-gi-base (>= 0.22) to access disconnectSignalHandler.

Since I changed the type of signalAddHandler this PR isn't backwards compatible. If you prefer to take a more compatible approach, like leaving the type of signalAddHandler unachanged and introducing the new type as something like signalAddHandler' or similar, just say so.

[mr]

I like this but is there a reason you bumped the min version?

[Kritzefitz]

I like this but is there a reason you bumped the min version?

disconnectSignalHandler was introduced in haskell-gi-base-0.22.0.

[Kritzefitz]

Heads up: I rebased my branch onto the current master, so you might have to git reset --hard if you have a local checkout of my branch.