applet/swkbd: Implement optional symbol keys

These are only used in the numeric keyboard, and correspond to the keys to the left and right of the "0" key on the numeric keyboard.

src/core/frontend/applets/software_keyboard.h

@@ -17,6 +17,8 @@ struct KeyboardInitializeParameters {
     std::u16string sub_text;
     std::u16string guide_text;
     std::u16string initial_text;
+    char16_t left_optional_symbol_key;
+    char16_t right_optional_symbol_key;
     u32 max_text_length;
     u32 min_text_length;
     s32 initial_cursor_position;