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/style.css

@@ -44,6 +44,17 @@ sq_button.change-view-2:active {
     border-color: alpha(@theme_fg_color, 0.275);
 }
 
+sq_button.placeholder,
+sq_button.placeholder-2 {
+    background: alpha(@theme_fg_color, 0.035);
+}
+
+sq_button.placeholder:active,
+sq_button.placeholder-2:active {
+    background: initial;
+    box-shadow: initial;
+}
+
 sq_button.altline,
 sq_button.special,
 sq_button.wide {