In the composer, when the text cursor was inside a word starting
with a @ but not matching any username (thus the completion UI being
invisible), pressing up/down arrow would not do anything instead of
going to the prev/next line.
Simplify the code, make it more declarative, and improve the reliability
of sending typing notifications (notably when changing the used alias in
the middle of a message).
thumbnail() and download() were remaining.
Use a direct and carefully chosen MatrixClient's methods instead to
avoid problems mentioned in the previous commit 7502c1.
