popup-menu: Add combo box menu item

Introduce a new menu widget, which displays the active item from
a set of options, and pops up a child menu to allow changing the
active item when activated.

https://bugzilla.gnome.org/show_bug.cgi?id=652837

data/theme/gnome-shell.css

@@ -139,6 +139,15 @@ StTooltip StLabel {
     border-width: 0px;
 }
 
+.popup-combo-menu {
+    background-color: rgba(0,0,0,0.9);
+    padding: 1em 0em;
+    color: #ffffff;
+    font-size: 10.5pt;
+    border: 1px solid #5f5f5f;
+    border-radius: 9px;
+}
+
 /* The remaining popup-menu sizing is all done in ems, so that if you
  * override .popup-menu.font-size, everything else will scale with it.
  */
@@ -158,6 +167,10 @@ StTooltip StLabel {
 .popup-image-menu-item {
 }
 
+.popup-combobox-item {
+    spacing: 1em;
+}
+
 .popup-separator-menu-item {
     -gradient-height: 2px;
     -gradient-start: rgba(8,8,8,0);