Fix text fields atop compose views.

The text fields would not perch atop the text view until after typing a newline
in the text view.
Make compose text view's cursor visible.
Make composition text views usable again.

They don't animate frame changes when the keyboard comes and goes though.
Fix forum tree controller's hidden forum set creation and management.

Rows are inserted out-of-order. We need to compute the hidden forum set for the
entire section once it's inserted in order to figure out where inserted rows
are visible.

This issue arose consistently on login, after the forum tree controller is
initialized, when all the forums get inserted as a batch.
Fix forum tree's notifying delegate of a deleted section.