Better support for the List class

[ameliatastic]

Python has a lot of methods implemented for its builtin list type that Seahorse doesn't yet support. Ideally, Seahorse should have parity with Python's list API, in order to make things as smooth as possible for developers.

Everything that needs to be support is listed here ("Lists implement all of the common and mutable sequence operations...").

(Putting all of this into one PR would be a huge lift, so we should split this up into smaller issues.)