frontend/applets: frontend swkbd base

Original commits by @jroweboy:

* Rebase out the other commit

* changing branches

* More work on stuff and things ecks DEE

Changes by @zhaowenlan1779:

* Removed #include of result.h

src/core/frontend/applet/interface.cpp

@@ -0,0 +1,20 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <unordered_map>
+#include "core/frontend/interface.h"
+
+namespace Frontend {
+
+std::unordered_map<AppletType, std::shared_ptr<AppletInterface>> registered_applets;
+
+void RegisterFrontendApplet(std::shared_ptr<AppletInterface> applet, AppletType type) {
+    registered_applets[type] = applet;
+}
+
+void UnregisterFrontendApplet(AppletType type) {
+    registered_applets.erase(type);
+}
+
+} // namespace Frontend