MSIX: implement initial version of notifications library (#1178)

src/common/notifications.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <string_view>
+#include <vector>
+
+namespace notifications
+{
+    void register_background_toast_handler();
+
+    // Make sure your plaintext_message argument is properly XML-escaped
+    void show_toast(std::wstring_view plaintext_message);
+    void show_toast_background_activated(std::wstring_view plaintext_message, std::wstring_view background_handler_id, std::vector<std::wstring_view> plaintext_button_labels);
+}