expo: Add spacing around menus and items

It looks better if menus have a bit of an inset, rather than be drawn hard
up against the background. Also, menu items look better if they have a bit
of spacing between them.

Add theme options for these and implement the required changes.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index ff9f922..38d5739 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -96,6 +96,8 @@
 
 		theme {
 			font-size = <30>;
+			menu-inset = <3>;
+			menuitem-gap-y = <1>;
 		};
 
 		/*