Light themes should disable the gtk3 backported resize grip

[Ric Flomag]

Binary package hint: light-themes

Rationale:
 * Ambiance and Radiance theme now have a 5px invisible resize grip all around windows, which makes the backported corner resize grip unnecessary.
 * The backported resize grip overlaps any element located at the right bottom corner of the window, like scrollbar buttons, other buttons, text, and so on... This affects the functionality and appearance of many applications. To name a few, Banshee, Firefox, Thunderbird, Software center... Other applications have been patched recently (Synaptic, the Appearance preferences...).
 * The effort to patch the applications one by one (as apparently planned at https://launchpad.net/hundredpapercuts/+milestone/nt9-grippy) will probably not solve the problem entirely (ie. for all applications).

Suggested solution:
 * Disable the corner grip in light themes.
 * Enable the invisible 5px grip for all themes and remove completely the gtk3 backport would be ideal.