Added missing "Search on..." translation string

I noticed the label "Search on..." was missing gettext translation so I simply changed it to "_("Search on…")".

data/gtk/window.blp

@@ -566,7 +566,7 @@ menu add_games {
 
 menu search {
   section {
-    label: "Search on…";
+    label: _("Search on…");
     item (_("IGDB"), "app.igdb_search")
     item (_("SteamGridDB"), "app.sgdb_search")
     item (_("ProtonDB"), "app.protondb_search")