Expose form fields as AtkObject children of the page

https://bugzilla.gnome.org/show_bug.cgi?id=728475
Setting parent for annotation properties dialog

After the last gtk+ update, the warning:

"Gtk-Message: GtkDialog mapped without a transient
parent. This is discouraged."

was shown whenever the annotation properties
dialog was opened. The commit fixes this issue by
setting this dialog transient for the main window.
As a plus, the dialog is now shown always on the
center of the main window.

https://bugzilla.gnome.org/show_bug.cgi?id=732114
Removing multi-screen support

Display-screen relation in GTK 3.10 is 1:1
(see https://mail.gnome.org/archives/gtk-list/2013-September/msg00021.html)
Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=701883
Removing function get_num_monitors
Removing function ev_display_open_if_needed

No longer used after support for multi-screen was
removed.