workspace: Use all available space for windows in window selector

Change the layout strategy to be more like the mockups. With less than
two rows of windows, we try to fit every window in a non-aligned situation;
with more than three rows of windows, we try to fit every window in an
aligned situation.

Based heavily on a patch from Pierre-Eric Pelloux-Prayer <pelloux@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=582650

data/theme/gnome-shell.css

@@ -640,6 +640,11 @@ StScrollBar StButton#vhandle:active {
     -st-background-image-shadow: 2px 2px 6px rgba(0,0,0,0.5);
 }
 
+.window-picker {
+    -horizontal-spacing: 40px;
+    -vertical-spacing: 40px;
+}
+
 /* Dash */
 
 #dash {