avocado.core.output: Introduce 'partial' event to View.notify()

And replace the last usage of the .log_ui methods in the
app code.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.core.output: View: Turn log_ui methods all private

Moving forward, we want users of the view API to use
only the notify methods. Therefore, turn all of them
to private.

In this process, we discovered one place that wasn't
using .notify (datadir plugin) and updated it.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.core.output: View: Refactor throbber related code

Take throbber update logic out of View into its own
Throbber class, and use that class as one attribute
of View. Makes up for a cleaner separation of attributions.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>