system: Align submenu labels with parent

All section titles use an icon, while all corresponding submenu items
are plain-text. As a result, labels in submenus aren't aligned with
the labels in the parent, which makes them harder to read and look
unbalanced. Address this by adding additional whitespace to submenu
items to account for the additional elements in the title.

https://bugzilla.gnome.org/show_bug.cgi?id=706191

data/theme/gnome-shell-high-contrast.css

@@ -987,6 +987,14 @@ StScrollBar {
   min-width: 21em; }
   .aggregate-menu .popup-menu-icon {
     padding: 0 4px; }
+  .aggregate-menu .popup-sub-menu .popup-menu-item :first-child:ltr {
+    /* 12px spacing + 2*4px padding */
+    padding-left: 20px;
+    margin-left: 1.09em; }
+  .aggregate-menu .popup-sub-menu .popup-menu-item :first-child:rtl {
+    /* 12px spacing + 2*4px padding */
+    padding-right: 20px;
+    margin-right: 1.09em; }
 
 .system-menu-action {
   color: #fff;

data/theme/gnome-shell-sass

@@ -1 +1 @@
-Subproject commit 32361ac692945e4c2c3ff80d20a4b03649699828
+Subproject commit 57a2e5bfe179d9db1e05c3edaffdcb3fee307be0

data/theme/gnome-shell.css

@@ -987,6 +987,14 @@ StScrollBar {
   min-width: 21em; }
   .aggregate-menu .popup-menu-icon {
     padding: 0 4px; }
+  .aggregate-menu .popup-sub-menu .popup-menu-item :first-child:ltr {
+    /* 12px spacing + 2*4px padding */
+    padding-left: 20px;
+    margin-left: 1.09em; }
+  .aggregate-menu .popup-sub-menu .popup-menu-item :first-child:rtl {
+    /* 12px spacing + 2*4px padding */
+    padding-right: 20px;
+    margin-right: 1.09em; }
 
 .system-menu-action {
   color: #eeeeec;