List item change to depend on TextRun

Following the issue #230 I have changed the inheritance of ListItem. It now inherits from TextRun, such that it behaves as a container. It is now possible to provide inline formatting of a ListItem.

@ivanlanin: I already appologize for not having my branch full in sync. I am still struggling with Eclipse, Egit and GitHub. I am able to start with a fully synced branch, but when changes are merged in the upstream branch, while I am coding, I am getting out of sync.
Still searching the web for a good tutorial :/