Add a service for snapshotting webpages

This service provides pixbufs for URLs while caching these locally as
thumbnails. gnome-desktop-thumbnail is used to handle caching.

https://bugzilla.gnome.org/show_bug.cgi?id=668578
Add basic tests for EphySnapshotService

https://bugzilla.gnome.org/show_bug.cgi?id=668578
Add GdMainView for use in the overview

This widget courtesy of gnome-documents

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Add the base EphyOverviewStore model

This should be subclassed by the models for the history and/or
currently open views.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
EphyOverviewStore: add ephy_overview_store_peek_snapshot()

This method takes care of loading a snapshot from the service,
asking for a new version when needed.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Add EphyActiveStore class files

This EphyOverviewStore subclass is fed by a EphyNotebook and keeps track
of changes in the pages that are part of it.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Add EphyFrecentStore class files

This subclass of EphyOverviewStore displays the most frecently visited
pages.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Add EphyOverview widget

This widget entails two GdMainViews, one for the frecency model and
one for the active model.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Add a boolean overview-mode property to EphyWindow

With this, add the EphyOverview widget to the main_vbox, so that
it's visible instead of the notebook when the property is TRUE.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Add a "Pages" button to the toolbar

This button enables the overview mode in the window.

https://bugzilla.gnome.org/show_bug.cgi?id=455173
Hide the overview on url load

By handling webview's load-status

https://bugzilla.gnome.org/show_bug.cgi?id=455173
ephy-window: show the overview on alt+home

https://bugzilla.gnome.org/show_bug.cgi?id=455173
ephy-window: make is-popup and chrome actual window properties

Removing them from EphyEmbedContainer.
Move implementation of EmbedContainer from Window to Notebook

The properties that made sense in window were already moved there
in the previous commit, everything else makes more sense in notebook.
ephy-window: only react to the active embed if the overview is hidden

When showing the overview, the window doesn't need to be updated with
changes in the currently active embed.
ephy-window: make default actions unsensitive when showing the overview
ephy-window: cleanup the ui before showing the overview

A few UI items are not needed when the window is displaying the overview,
clean them up.
ephy-window: hide favicon when the overview is visible
gschema: hide tabs by default

The overview doesn't need them
ephy-removable-pixbuf-renderer: new renderer for the overview

This is a renderer that draws a "close" button on top of its contents
and that emits a signal when the button is activated.