workspace: Reduce spacing between window previews

With the window titles no longer being shown as part of the previews
grid, we can reduce the spacing to have more space available to the
window previews themselves.

https://bugzilla.gnome.org/show_bug.cgi?id=783953

data/theme/gnome-shell-high-contrast.css

@@ -1094,13 +1094,11 @@ StScrollBar {
   padding-bottom: 32px; }
 
 .window-picker {
-  -horizontal-spacing: 32px;
-  -vertical-spacing: 32px;
-  padding-left: 32px;
-  padding-right: 32px;
-  padding-bottom: 48px; }
+  -horizontal-spacing: 16px;
+  -vertical-spacing: 16px;
+  padding: 0 16px 16px; }
   .window-picker.external-monitor {
-    padding: 32px; }
+    padding: 16px; }
 
 .window-clone-border {
   border: 4px solid #215d9c;

data/theme/gnome-shell.css

@@ -1094,13 +1094,11 @@ StScrollBar {
   padding-bottom: 32px; }
 
 .window-picker {
-  -horizontal-spacing: 32px;
-  -vertical-spacing: 32px;
-  padding-left: 32px;
-  padding-right: 32px;
-  padding-bottom: 48px; }
+  -horizontal-spacing: 16px;
+  -vertical-spacing: 16px;
+  padding: 0 16px 16px; }
   .window-picker.external-monitor {
-    padding: 32px; }
+    padding: 16px; }
 
 .window-clone-border {
   border: 4px solid #215d9c;