Remove textIsSelectable attribute from the conversation input EditText.

Fix the bug which prevents the soft keyboard from appearing when user
clicks on the input EditText.
Replace the Disconnect menu by Connect when the server is disconnected.
Refactoring of MessageListAdapter: replace TextView by SpannableString.

TextView are far more expensive than SpannableString to be cached.
Caching the views don't worth it as ListView already allows to easily
reuse the views. It virtually improves no performance, but it's costly in
memory.