[Merge] lp:~uriboni/webbrowser-app/newtabview-listviews into lp:webbrowser-app

[Ugo Riboni]

> If we want to optimize for large bookmark databases, this branch doesn’t
> appear to be improving things, quite the contrary. All of this is anecdotal
> when we look at the time it takes for the application itself to launch and
> become responsive though. With 100000 bookmarks, the app takes ~20s to launch [when the new tab view is visible]

The problem was that the collapsed bookmarks view was still a Column+Repeater+Loader and that was causing the long startup. This has been solved by use of a ListView backed by a LimitProxyModel. 

With this change, I can't find any major bottleneck that prevent the app to be usable in any condition.
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https://code.launchpad.net/~uriboni/webbrowser-app/newtabview-listviews/+merge/274986
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