[client] app: delay showing the escape help for 250ms

This change prevents the help text being shown unless the escape key has
been held for a minimum of 250ms (reduced from 500ms).

client/include/app.h

@@ -54,6 +54,7 @@ void app_handleKeyRelease(int scancode);
 void app_handleEnterEvent(bool entered);
 void app_handleFocusEvent(bool focused);
 void app_handleCloseEvent(void);
+void app_handleRenderEvent(const uint64_t timeUs);
 
 void app_setFullscreen(bool fs);
 bool app_getFullscreen(void);