Add SDL_WINDOW_NOT_FOCUSABLE flag to set that the window should not be able to gain key focus

- Also adds SDL_SetWindowFocusable() to set/clear flag on existing windows

src/dynapi/SDL_dynapi.sym

@@ -901,6 +901,7 @@ SDL3_0.0.0 {
     SDL_WriteS64LE;
     SDL_WriteS64BE;
     SDL_GDKGetDefaultUser;
+    SDL_SetWindowFocusable;
     # extra symbols go here (don't modify this line)
   local: *;
 };