fix: performance-enum-size warnings (#6504)

gtk/FilterBase.h

@@ -15,12 +15,14 @@
 #include <glibmm/object.h>
 #endif
 
+#include <cstdint>
+
 template<typename T>
 class FilterBase : public IF_GTKMM4(Gtk::Filter, Glib::Object)
 {
 public:
 #if !GTKMM_CHECK_VERSION(4, 0, 0)
-    enum class Change{
+    enum class Change : uint8_t{
         DIFFERENT,
         LESS_STRICT,
         MORE_STRICT,