[Merge] lp:~artmello/webbrowser-app/webbrowser-app-bookmarks_top_sites into lp:webbrowser-app

[Arthur Mello]

> As already discussed, in the current implementation TopSitesSheet is always
> instantiated. Instead, it should be loaded on demand (encapsulating it in a
> Component, or using a Loader), only when the user requests a new tab. It
> should probably be destroyed as soon as the URL of the current webview becomes
> non empty, and instantiated again when switching to a webview with an empty
> URL.

Fixed on revision 571
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https://code.launchpad.net/~artmello/webbrowser-app/webbrowser-app-bookmarks_top_sites/+merge/224592
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