style: update panel CSS for Gtk.MenuButton

Add #global-menu reset rules, hide indicator, and transparent
:active state for global menu and panel buttons.

src/styles/window/panel.css

@@ -34,6 +34,19 @@
     margin: 5px 2px;
     padding: 0 5px;
     border-radius: apply(radius-xs);
+    background: none;
+    border: none;
+    outline: none;
+    min-height: 0;
+    min-width: 0;
+}
+
+#global-menu indicator {
+    min-width: 0;
+    min-height: 0;
+    margin: 0;
+    padding: 0;
+    -gtk-icon-source: none;
 }
 
 #global-menu:active {