Fix the Triage Summary view's exports list to only update when visible

[bdash]

The ExportsTreeView now asks the model to pause / resume updates when its visibility changes. When paused, all notifications from the view are ignored. When resumed, notifications set a flag to indicate that an update is needed and resume the update timer if it is not already active. The timer is stopped after an update is processed.

There's some extra complexity here to avoid emitting a signal for every BinaryView notification that is processed. These notifications are typically generated on background threads. The overhead of emitting a signal and it being routed to the main thread adds up given the number of notifications involved when loading a large binary. This manifests on macOS as time being spent inside CFRunLoopWakeUp.

[bpotchik]

Implementation looks good.

Additional context (informational only):

SidebarWidgets and Views include a quiescing interface (enableRefreshTimer) that provides a base timer and control signals for when widgets are shown/hidden in the UI. The current approach using showEvent/hideEvent works well for this scenario since the widget resets the model rather than collecting data from each BinaryDataNotification.

Additionally, it's possible to completely unregister from BinaryViewNotifications when the widget is hidden, then re-register and rebuild the model when it becomes visible again. I only mention this because, occasionally when opening a file and view navigation fails, the triage view is gratuitously created but never shown. It would then be an idle consumer of all notifications it registered for. There are some nuances to implementing this, so just providing it as information.

[plafosse]

I'm approving but I left a couple of comments on some potential improvements. Address those comments if you agree with them and then merge.