config: make CSD user configurable

The user can now configure the following:

* Whether to prefer CSDs or SSDs. But note that this is only a hint to
  the compositor - it may deny our request. Furthermore, not all
  compositors implement the decoration manager protocol, meaning CSDs
  will be used regardless of the user configuration (GNOME/mutter
  being the most prominent one).
* Title bar size and color, including transparency
* Border size and color, including transparency

Also drop support for rendering the CSDs inside the main surface.

config.h

@@ -36,6 +36,20 @@ struct config {
         } color;
     } cursor;
 
+    struct {
+        enum { CONF_CSD_PREFER_SERVER, CONF_CSD_PREFER_CLIENT } preferred;
+
+        int title_height;
+        int border_width;
+
+        struct {
+            bool title_set;
+            bool border_set;
+            uint32_t title;
+            uint32_t border;
+        } color;
+    } csd;
+
     size_t render_worker_count;
     char *server_socket_path;
     bool presentation_timings;