Override context menu for links, images, videos and selection (#7)
Especially noteworthy items here are search (DuckDuckGo hard-coded for now, just like search from the urlbar) and new private browsing window, other items are being overridden mainly for the sake of a clean implementation.
More consistent handling of Tab.item and Tab.create (#8)
* create normally loads a given request and emits ready_to_show, so we should support this and emulate the same behavior when emitting create.
* a new browser tab should get focus.
* Tab.item should be set in the case of a delayed load.
* tooltips should always be set for the tally.
Tabs should only take the URI to be loaded as "display-uri" and not actually load it until getting focus. To make this look smooth, the title is looked up in the history (the URI will be taken from the favicon database automatically as usual).