Fix checkboxes, radiobuttons and corresponding menu items.

Use -gtk-icon-source instead of background-image.
Specify proper size of icons, otherwise images appear streched.
Recrop checkbox images to remove empty space around the check marks.
Set the default background for .background only.
Use built-in GTK+ animated spinner.
Fix active/focus/checked buttons.
Combobox entries are no longer GtkComboBox.
Flat buttons are .flat now.
Link .linked buttons and combos together.
Popovers need background!
Better menuitem handling.
Active menu button needs a border.
Better padding for buttons.
Notebook borders and backgrounds.
This code doesn't work properly anyway, disable it.