Add sections to the all apps view

Separate out the main app view into different sections based on the categories
in the desktop file. The configuration is done via gmenu and the desktop menu
specification, we set XDG_MENU_PREFIX="gs-" on startup, so that gmenu reads
gs-applications.menu, which we install.

There is no support for "submenus" - only the menus directly under
Applications will be displayed as categories.
https://bugzilla.gnome.org/show_bug.cgi?id=614131

data/theme/gnome-shell.css

@@ -520,6 +520,16 @@ StTooltip {
     color: #ffffff;
 }
 
+.app-section-divider-container {
+    padding-top: 10px;
+    padding-bottom: 10px;
+}
+
+.app-section-divider {
+    height: 2px;
+    background-image: url("separator-white.png");
+}
+
 .all-app-controls-panel {
     height: 30px;
 }