frontend/applets: Add frontend software keyboard provider and default

Default implementation will return "yuzu" for any string. GUI clients (or CLI) can implement the Frontend::SoftwareKeyboardApplet class and register an instance to provide functionality.

src/core/frontend/applets/software_keyboard.h

@@ -0,0 +1,44 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <string>
+#include "common/bit_field.h"
+#include "common/common_types.h"
+
+namespace Frontend {
+class SoftwareKeyboardApplet {
+public:
+    struct Parameters {
+        std::u16string submit_text;
+        std::u16string header_text;
+        std::u16string sub_text;
+        std::u16string guide_text;
+        std::u16string initial_text;
+        std::size_t max_length;
+        bool password;
+        bool cursor_at_beginning;
+
+        union {
+            u8 value;
+
+            BitField<1, 1, u8> disable_space;
+            BitField<2, 1, u8> disable_address;
+            BitField<3, 1, u8> disable_percent;
+            BitField<4, 1, u8> disable_slash;
+            BitField<6, 1, u8> disable_number;
+            BitField<7, 1, u8> disable_download_code;
+        };
+    };
+
+    virtual bool GetText(Parameters parameters, std::u16string& text) = 0;
+    virtual ~SoftwareKeyboardApplet() = default;
+};
+
+class DefaultSoftwareKeyboardApplet final : public SoftwareKeyboardApplet {
+    bool GetText(Parameters parameters, std::u16string& text) override;
+};
+
+} // namespace Frontend