Instead of focusing the composer, focus the buffer scrollview when
switching to a buffer. This allows keyboard navigation to work as
expected, with arrow up/down and page up/down scrolling the buffer
instead of doing nothing.
Focus back the composer when a KeyboardEvent produces text. This
allows users to start typing a message right after switching to a
buffer.
Closes: https://todo.sr.ht/~emersion/gamja/64
724d7318cf