graphview: draw background automatically

This removes the manual painting of the background via cairo and adds the correct color to CSS instead,
which should hopefully improve performance as we do less cpu painting like this.

src/style.css

@@ -2,7 +2,6 @@
 @define-color video rgb(200,200,0);
 @define-color midi rgb(200,0,50);
 @define-color graphview-link #808080;
-@define-color graphview-grid @text_view_bg;
 
 .audio {
     background: @audio;
@@ -17,4 +16,8 @@
 .midi {
     background: @midi;
     color: black;
+}
+
+graphview {
+    background: @text_view_bg;
 }