libview: Rename ev_view_set_focused_element and make it public internally

This will be needed in order to implement atk_component_grab_focus

https://bugzilla.gnome.org/show_bug.cgi?id=728475
ev-page-cache: Add method to check if a given page has been cached

The Accessibility code will expose child objects (links, images, form
fields) of the page, but can only do so after that page has been cached.

https://bugzilla.gnome.org/show_bug.cgi?id=728475
ev-view-accessible: Add a method to determine if a given doc rect is showing

The Accessibility code needs to know this to set and update ATK_STATE_SHOWING.

https://bugzilla.gnome.org/show_bug.cgi?id=728475
Expose links as AtkObject children of the page

https://bugzilla.gnome.org/show_bug.cgi?id=728475