endSessionDialog: prelight inhibiting app items

This commit makes it more obvious that apps in the end session
dialog inhibitors list are clickable.  It does this by rendering
the text for the apps in a low intensity white under normal
conditions, but a high intesnity white on hover.

https://bugzilla.gnome.org/show_bug.cgi?id=645491

data/theme/gnome-shell.css

@@ -1474,6 +1474,14 @@ StTooltip StLabel {
     padding-left: 32px;
 }
 
+.end-session-dialog-app-list-item {
+    color: #ccc;
+}
+
+.end-session-dialog-app-list-item:hover {
+    color: white;
+}
+
 .end-session-dialog-app-list-item:ltr {
     padding-right: 1em;
 }