more gtk3 theming

Continuing on the Gtk3 theming work. Now got the combobox and editbox rendering and sizes correct along with new gtk3-alike focus rectangles. Here's the after...

Here's the before of what the gtk3 effort looked like in 4-4

Here's the equivalent 4-4 gtk2 effort. Note that now in the above gtk3 theming we have a single focus rectangle for the full combobox rather than a focus rectangle around the non-button part of the widget and that, as in a normal gtk3 combobox, the background isn't set to blue when selected. I always hated that out of character blue listbox/combobox selection color. So certain elements of the gtk3 theming now slightly surpass the gtk2 one which is nice. Though clearly the spinbuttons are still effectively imaginary ones as they look nothing like the native gtk3 ones.

I also fixed (for both gtk2 and gtk3) that notorious checkbox issue where unchecking a checkbox would leave a portion of the check still drawn outside the checkbox rectangle.