TraceView: Improve the way we do the initial scrolling setup

TraceView: Improve the way we do the initial scrolling setup

The two commits
ae5f66281c38a76dcb7011907d7a86a99b9dd4dd and
66e4eae9ce743016c3f6d4974bceb9b7fc0ae7d7

Were okay but it showed that there was one case they didn't
cover: the first session receives a resize notification upon
startup whereas all other sessions don't. This means that
sessions restored from a previous run suddenly see the
size_finalized_ variable always staying at false - until the
user resizes the window.

While figuring out how to cover this case, I realized that
there is actually an easier way to perform all this:
instead of keeping an internal state of when and how we
received useless and useful resize notifications, we just
act upon the "show" event, which always comes after all
the widget resizing has been performed. This way, we can
remove the size_finalized_ variable completely and as it
is definitely always received when a session is shown,
we always end up in a sane and correct state.

Note: to reproduce, open up a 2nd session and use demo.
Then, when using the group handle to move the analog traces
upwards, you'll notice that the logic traces will move down
at the same time. This is because size_finalized_ is false
and the view will always center the traces vertically.