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
Fixing annotation window colors

The annotation window now is all of one color and
no border.
Improving annotation window appearance

The title is now bold and there is extra margin
space around the textbox.
Also fixing minor typo.
Closing annotation window by clicking on icon

When the annotation window is opened, clicking on
the corresponding icon closes it.