add demo checkAbort

Author: Remi-Gau

demos/CPP_checkAbortDemo.m

@@ -0,0 +1,12 @@
+cd ..
+
+cfg.keyboard = [];
+cfg.escapeKey = 'ESCAPE';
+KbName('UnifyKeyNames');
+
+% stay in the loop until the escape key is pressed
+while GetSecs < Inf
+    
+    checkAbort(cfg)
+    
+end