StyleCSS: abandon using a theme engine

In GTK-2 you'd always use a theming engine.
During the transitional period, GTK-3 followed that path,
but at the point when the new CSS based system was mature enough,
this approach was discouraged, since it means to ship additional
executable code and defeats the purpose of configuring the look
of the application through one coherent set of rules.

As it turns out, debian continued to ship the 'adwaita' for some
extended transitional period, but in Stretch it is gone, which broke
our half-way ported stylesheet.

This changeset removes the use of a theming engine, and fixes tha
basic look by adding the background-color: inherit, which pushes aside
many (but not all) of the default theme definitions.

However, our stylesheet remains mostly broken and TODO for now

src/gui/gtk-lumiera.css

@@ -35,13 +35,12 @@
 
 
   * {
-    engine: adwaita;
-    
     border-width: 1px;
     border-style: outset;
     border-color: shade(@bg_color, 0.80);
     
-    color: inherit; /* by default inherit content colour from parent widget */
+    color:            inherit; /* by default inherit content colour from parent widget */
+    background-color: inherit;
 }
 
 GtkWindow {

src/gui/workspace/ui-style.cpp

@@ -96,7 +96,7 @@ namespace workspace {
       }
     
     Gtk::StyleContext::add_provider_for_screen (screen, css_provider,
-                                                GTK_STYLE_PROVIDER_PRIORITY_USER);
+                                                GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
   }