directly embed SVGs for better styling

Now the SVGs are loaded via AJAX and are directly embedded into the
HTML. This allows for styling them via CSS and we can apply proper hover
styles.

The svg dispatcher was extended with an embed option (boolean parameter
'e') which will return an absolute minimum svg with absolutely no
styles.

css/area_main-sidebar.less

@@ -35,10 +35,12 @@
                 margin-right: @margin-small;
                 color: @color-nav;
 
-                img {
+                svg {
                     width: @icon-size;
                     height: @icon-size;
-                    filter: grayscale(100%); //fixme maybe we could find a better way
+                    path {
+                        fill: @color-nav;
+                    }
                 }
             }
 
@@ -50,8 +52,8 @@
                 color: @button_background;
                 text-decoration: none;
 
-                span img {
-                    filter: none;
+                span svg path {
+                    fill: @button_background;
                 }
             }
         }