GtkSettings: fully undeprecate blink settings

We decided in f8412eca34be6e2e655ad3e09bd97fa2bdcbdae0 that
we still need to react to these for a11y reasons, but left
the (then) harmless property deprecation in place. Now, the
deprecation causes runtime warnings for merely reading the
property, so drop it.

https://bugzilla.gnome.org/show_bug.cgi?id=732667
docs: make GtkWidget:margin-start and margin-end clearer

margin-start and margin-end have been added for RTL support, see:
https://bugzilla.gnome.org/show_bug.cgi?id=710238

So those properties are used only for the horizontal direction: left or
right, not top or bottom.

https://bugzilla.gnome.org/show_bug.cgi?id=732681