Provide more abstraction for overlays and float panes

[illright]

They provide similar functionality, are similar in appearance, suffer from the same issues and even have similar codebases. I believe we could be more efficient at maintaining both datatip and signature help functionality if we combine it in a single package that provides two services.

Additionally, this one is now TypeScript'ified, while atom-ide-signature-help isn't.

[lierdakil]

This might make sense, with one caveat: signature help tooltip is similar to a datatip, but it has some distinct behaviours. In particular, you generally don't have dozens of datatips, but you can have dozens of signature helps (for overloaded functions). Being able to show one at a time and cycle through is a must (which is by the way something atom-ide didn't do particularly well)

[aminya]

As mentioned, they are different. The common code is already refactored and lives in atom-ide-base. Both use ViewContainer and ProviderRegister. But the remaining code is different.

We can make another abstraction on top of this, but I don't think it would be helpful.