Workspace: Highlight window clone and caption when hovered

Windows in the overview should be highlighted when hovered, to indicate
they are an active target.

Based on a patch by Marc Plano-Lesay <marc.planolesay@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=665310

data/theme/gnome-shell.css

@@ -615,6 +615,7 @@ StScrollBar StButton#vhandle:active {
     border-radius: 8px;
     padding: 4px 12px;
     -shell-caption-spacing: 12px;
+    border: 2px solid rgba(0, 0, 0, 0);
 }
 
 .window-close, .notification-close {
@@ -625,7 +626,16 @@ StScrollBar StButton#vhandle:active {
 }
 
 .window-close {
-    -shell-close-overlap: 20px;
+    -shell-close-overlap: 16px;
+}
+
+.window-caption:hover {
+    border: 2px solid rgba(255, 255, 255, 0);
+}
+
+.window-clone-border {
+    border: 4px solid rgba(255, 255, 255, 0.5);
+    border-radius: 4px;
 }
 
 .notification-close {