theme: Restrict the aggregate menu to 320px

Like in the designs. This also requires that we don't put a min-width
on sliders, as 15em is smaller than 320px.

https://bugzilla.gnome.org/show_bug.cgi?id=705845

data/theme/gnome-shell.css

@@ -127,7 +127,6 @@ StScrollBar StButton#vhandle:active {
 
 .slider {
     height: 1em;
-    min-width: 15em;
     -slider-height: 0.3em;
     -slider-background-color: #333333;
     -slider-border-color: #5f5f5f;
@@ -631,6 +630,10 @@ StScrollBar StButton#vhandle:active {
     icon-size: 1.09em;
 }
 
+.aggregate-menu {
+    width: 320px;
+}
+
 .system-switch-user-submenu-icon {
     icon-size: 24px;
     border: 1px solid #8b8b8b;