More GSoC ideas

The ideas list was looking a bit lacklustre, so I've written up some of the ones mentioned in the dev group, as well as trying to make things a bit more organised. The two ideas that were already there I've left alone, primarily to avoid stepping on toes - but I can rewrite things to make them more consistent if wanted.

My impression is that the ideas page is a fairly important part of the application process, so I'll continue to improve it where I can.