gui2: added generic button class

this gives child elements a button behavior.

avr/modules/lcd2/gui2/base/object.asm

@@ -337,19 +337,19 @@ OBJ_GetLastChild_ret:
 ; @param R16 idx of child to get (starting at 0)
 ; @return CFLAG set, if found, cleared otherwise
 ; @return r19:r18 resulting object
-; @clobbers none
+; @clobbers r16
 
 OBJ_GetChildAt:
   push yl
     push yh
       rcall OBJ_GetFirstChild     ; R19:R18=obj (none)
       brcc OBJ_GetChildAt_ret
-
 OBJ_GetChildAt_loop:
         tst r16
         breq OBJ_GetChildAt_secRet
         mov yl, r18
         mov yh, r19
+        dec r16
         rcall OBJ_GetNext         ; R19:R18=obj (none)
       brcs OBJ_GetChildAt_loop
       rjmp OBJ_GetChildAt_ret     ; idx too high, not found