Add touch began to pan gesture

!!!!!!一定要做#import "UIPanGestureRecognizerSubclass.h"
然後用category或是subclass去設定這個state，
才能用readwrite不然，原本的recognizer是readonly










@interface TouchableImageView:UIImageView
@property(strong, nonatomic) UIPanGestureRecognizer *panGR; // 這裡要改成自己的recognizer或是實作一個category
@end

@implementation TouchableImageView
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    self.panGR.state = UIGestureRecognizerStateBegan;
}
@end
//直接改狀態就能trigger gesture的began!!