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/doc/develop/expo.rst b/doc/develop/expo.rst
index 80e435c..bd593dc 100644
--- a/doc/develop/expo.rst
+++ b/doc/develop/expo.rst
@@ -162,6 +162,12 @@
 font-size
     Font size to use for all text (type: u32)
 
+menu-inset
+    Number of pixels to inset the menu on the sides and top (type: u32)
+
+menuitem-gap-y
+    Number of pixels between menu items
+
 
 API documentation
 -----------------