Since the selection already gets updated in AbstractView::keydown_event
and AbstractView::mousedown_event we don't have to update it again in
the MainWidget::open_url function.
This fixes a bug that causes the selection to "spaz out" when
scrolling too fast between pages.
Up and down arrows now select search results. Matching index now
displays in full before filtering. Searching from the command
line focuses the query, and searches are now case insensitive.
Converts Help's layout to GML, propagates model and icon creation
errors, and switches to the MainWidget-namespace organization
pattern seen in more up-to-date apps like TextEditor to make things
easier to maintain going forward.