Show drafts in ConversationList

I modified ThreadDatabase.update() to show an italic "Draft" in the ConversationList, when a draft exists. This way threads with drafts are shown on the top (sorted by date) and drafts for new recipients don't show the default "(No subject)" text. I'm not sure if I implemented this the correct way. I added a new constant MESSAGE_DRAFT_BIT in MmsSmsColumns.Types to indicate the conversation contains a draft.

This PR is currently based on the PR #1510, but it doesn't depend on the changes in that PR, so it can also be rebased on master and committed independently.