[hle,display,overlay,starter,hid] add overlay functions, starter applet (initially), HID handheld for system applets and fw21 stubs (#3080)

Adds fully functional overlay display.
- Enable Overlay Applet via "View" -> "Enable Overlay Display Applet"
- Open the overlay by pressing the home button for over 1s
- Can adjust volume
- Can toggle airplane mode (if on WiFi, maybe if overlay is enabled pretend to be on WiFi?)
- Future TODO(?): Adjust Brightness implementation for host system
- Inputs are properly registered. e.g. if overlay open, application does not register inputs.

You can control volume and airplane mode outside of the emulator window

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3080
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>
Reviewed-by: crueter <crueter@eden-emu.dev>
Co-authored-by: Maufeat <sahyno1996@gmail.com>
Co-committed-by: Maufeat <sahyno1996@gmail.com>

src/core/core.h

@@ -17,6 +17,8 @@
 
 #include "common/common_types.h"
 #include "core/file_sys/vfs/vfs_types.h"
+#include "core/hle/service/os/event.h"
+#include "core/hle/service/kernel_helpers.h"
 
 namespace Core::Frontend {
 class EmuWindow;
@@ -428,6 +430,11 @@ public:
      */
     [[nodiscard]] std::deque<std::vector<u8>>& GetUserChannel();
 
+    [[nodiscard]] std::deque<std::vector<u8>>& GetGeneralChannel();
+    void PushGeneralChannelData(std::vector<u8>&& data);
+    bool TryPopGeneralChannel(std::vector<u8>& out_data);
+    [[nodiscard]] Service::Event& GetGeneralChannelEvent();
+
     /// Type used for the frontend to designate a callback for System to exit the application.
     using ExitCallback = std::function<void()>;