Make spacing between workspaces stylable

Currently the width of the gaps between workspaces in both linear
and mosaic view are defined as constants. Move these to the theme's
CSS instead.

https://bugzilla.gnome.org/show_bug.cgi?id=609673

data/theme/gnome-shell.css

@@ -162,6 +162,14 @@ StTooltip {
     color: white;
 }
 
+.workspaces.single {
+    spacing: 25px;
+}
+
+.workspaces.mosaic {
+    spacing: 15px;
+}
+
 .workspaces-bar {
     height: 48px;
 }