Make GtkCssProvider deal with widget types not being in plain CamelCase

Make GtkCssProvider deal with widget types not being in plain CamelCase

Fixes bug #Bug 639754, reported by Kjell Ahlstedt. gtkmm doesn't
use plain CamelCase for its widget type names, so in order to
distinguish widget type names from regions in the CSS parser,
the following checks are now done:

* if it contains an uppercase letter -> widget class (that should
  also work for gtkmm)

* if it's a string compound by lowercase letters and '-' -> it's
  a region, checks have been added in gtk_style_context_add_region()
  and gtk_widget_path_iter_add_region() to ensure this.