Keep track of previously installed builds and indicate them in the gui

[LS80]

First suggested at http://openelec.tv/forum/110-update-openelec/49855-addon-to-install-development-builds-dev-update?start=255#110466

[LS80]

I've made the first steps towards this by recording the install history with commit 523d409.

I haven't yet added a way to view this history.