Idempotent ring elements

[Alizter]

We should define what it means for a ring element to be idempotent and prove some basic properties about them.

We should also link this to decomposition of $R$-modules. if $e$ is an idempotent element of $R$ then an $R$-module $M$ can be decomposed as $M \cong eM \oplus (1-e)M$. This will be important for an eventual proof of the Wedderburn-Artin theorem.

[Alizter]

I think I will consider this issue resolved with #2105. The comment I made about decomposing modules should follow from a more general result about orthogonal idempotents (essentially giving a basis). That can only be stated once we have material on direct sums so no point keeping just this open for that.