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.h

@@ -0,0 +1,66 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <atomic>
+#include <memory>
+
+namespace Frontend {
+
+enum class AppletType {
+    SoftwareKeyboard,
+};
+
+class AppletConfig {};
+class AppletData {};
+
+class AppletInterface {
+public:
+    virtual ~AppletInterface() = default;
+
+    /**
+     * On applet start, the applet specific configuration will be passed in along with the
+     * framebuffer.
+     */
+    // virtual void Setup(const Config* /*,  framebuffer */) = 0;
+
+    /**
+     * Called on a fixed schedule to have the applet update any state such as the framebuffer.
+     */
+    virtual void Update() = 0;
+
+    /**
+     * Checked every update to see if the applet is still running. When the applet is done, the core
+     * will call ReceiveData
+     */
+    virtual bool IsRunning() {
+        return running;
+    }
+
+private:
+    // framebuffer;
+    std::atomic_bool running = false;
+};
+
+/**
+ * Frontends call this method to pass a frontend applet implementation to the core. If the core
+ * already has a applet registered, then this replaces the old applet
+ *
+ * @param applet - Frontend Applet implementation that the HLE applet code will launch
+ * @param type - Which type of applet
+ */
+void RegisterFrontendApplet(std::shared_ptr<AppletInterface> applet, AppletType type);
+
+/**
+ * Frontends call this to prevent future requests
+ */
+void UnregisterFrontendApplet(AppletType type);
+
+/**
+ * Returns the Frontend Applet for the provided type
+ */
+std::shared_ptr<AppletInterface> GetRegisteredApplet(AppletType type);
+
+} // namespace Frontend