Retain a number of messages when performing GC

Avoids behaviour described in #937 (closed) where messages can be repetitively detached and downloaded if they are outside the account download threshold but at the top of the folder.

This is currently set to retain a static 100 messages. I had a quick look at plumbing a hidden setting in for this, which I'd prefer, but it looked like it might take a little more time than I'd hoped for this quick fix. If there is a fairly quick and clean way to access a GSetting from within the engine I'm happy to give it a crack, otherwise this is (for me) ready to merge.

Closes #937 (closed)