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