Use thumbnails as default sidebar page

When the sidebar page set by the document metadata
is not supported, it falls back to thumbnails.

https://bugzilla.gnome.org/show_bug.cgi?id=628617
sidebar: Enable supported pages after reloading

When a document is reloaded, the supported pages
in the sidebar should be enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=628617
sidebar: select correct page after reloading

Once a document is reloaded, if the current
sidebar page is no longer supported, it should
change to the first supported page. Otherwise, it
does not change pages.

https://bugzilla.gnome.org/show_bug.cgi?id=628617
Finnish help translation
Updated Belarusian translation.

(cherry picked from commit f69d58ac81a6cc1e74bf7a0d7d78a0af3db42bf5)
libgd: Added libgd to cut-n-paste

A separate commit where libgd is added to cut-n-paste. libgd is used for the bookshelf
view of recent documents.
libgd: Use cairo surfaces instead of pixbufs in gd_embed_image_in_frame
libgd: Expect a cairo surface instead of a pixbuf for the GdMainIconView cell renderer
ev-recent-view: Added bookshelf view for recent documents

* Files are added to the bookshelf only if they exists
* Use gtk_recent_info_get_modified instead of
  gtk_recent_info_get_visited for ordering the recent items.
recent-view: ev_recent_view_new should return a GtkWidget
libgd: Highlight items on mouse-hover
recent-view: Do not propagate button_press_event in recent view

Otherwise, when using single clicks the drag icon would remain
active.
[l10n] Updated Catalan (Valencian) translation
Add a configure check for hi-dpi support

In order to support hi-dpi (GDK_SCALE) we need new-enough versions
of both GTK+ and cairo. Add a configure check for the appropriate
functions and define HAVE_HIDPI_SUPPORT.

https://bugzilla.gnome.org/show_bug.cgi?id=723431
ishell: Make the about dialog non modal
shell: Update website and copyright years in about dialog
shell: Move about dialog to the app menu and make it independent from the document
shell: Use a GtkHeaderBar instead of a GtkToolbar and window decorations

Based on patch by Yosef Or Boczko.

https://bugzilla.gnome.org/show_bug.cgi?id=711078
Revert "shell: Use a GtkHeaderBar instead of a GtkToolbar and window decorations"

This reverts commit c9127524e1409d582f038439e9143695d1fadc0d.

It broke the fullscreen mode.
recent-view: Verify a path exists before adding an icon

Fixes warnings when switching from the recent view to
a document back and forth.