win-capture: Use window for keepalive check

To check to make sure game capture is still active in the capture
program, it currently uses a named event, and then it checks to see if
that named event exists.  However with UWP programs, you can't open a
named event outside of the UWP process.  FindWindow on the other hand
does work, so instead of checking to see if a named kernel object
exists, create a window and check to see if that window exists.

plugins/win-capture/graphics-hook-info.h

@@ -13,7 +13,7 @@
 #define EVENT_HOOK_READY      "CaptureHook_HookReady"
 #define EVENT_HOOK_EXIT       "CaptureHook_Exit"
 
-#define EVENT_HOOK_KEEPALIVE  "CaptureHook_KeepAlive"
+#define WINDOW_HOOK_KEEPALIVE L"CaptureHook_KeepAlive"
 
 #define MUTEX_TEXTURE1        "CaptureHook_TextureMutex1"
 #define MUTEX_TEXTURE2        "CaptureHook_TextureMutex2"