Add placeholder-buttons to emoji-layout

to enhance contrast between usable and unusable buttons.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/636>

data/keyboards/emoji/us.yaml

@@ -1,11 +1,12 @@
 ---
 outlines:
     default:       { width: 52,   height: 52 }
+    placeholder:   { width: 52,   height: 52 }
     special:       { width: 52,   height: 52 }
     change-view:   { width: 52,   height: 52 }
     change-view-2: { width: 36.4, height: 56 }
     emoji-group:   { width: 52,   height: 52 }
-    infobar:       { width: 260,  height: 52 }
+    placeholder-2: { width: 260,  height: 52 }
 
 views:
     base:
@@ -854,10 +855,11 @@ buttons:
         icon: "edit-clear-symbolic"
         keysym: BackSpace
     InfoBar:
-        outline: "infobar"
+        outline: "placeholder-2"
         text: ""
         label: "🧘 + 🏾 = 🧘🏾"
     0:
+       outline: "placeholder"
        text: ""
        label: ""
     1: