[Fix] slideshow swipe wrong direction detection.

the current code was from old code without swipe support, now we has generic swipe detector, and swipe actions, so directly use it.

else on iOS, a fast swipe left will generate events like this:
```
2013-03-19 21:23:33.110 XBMC[4727:707] [DEBUG] -[XBMCController handleSwipe:]: [744.000000, 300.000000], direction: 2, touch num: 1
2013-03-19 21:23:33.114 XBMC[4727:707] [DEBUG] -[XBMCController handlePan:]: <UIPanGestureRecognizer: 0x4664d60; state = Began; delaysTouchesBegan = YES; view = <IOSEAGLView 0x46f6f00>; target= <(action=handlePan:, target=<XBMCController 0x46f9420>)>>
2013-03-19 21:23:33.122 XBMC[4727:707] [DEBUG] -[XBMCController handlePan:]: <UIPanGestureRecognizer: 0x4664d60; state = Changed; delaysTouchesBegan = YES; view = <IOSEAGLView 0x46f6f00>; target= <(action=handlePan:, target=<XBMCController 0x46f9420>)>>
2013-03-19 21:23:33.124 XBMC[4727:707] [DEBUG] -[XBMCController handlePan:]: <UIPanGestureRecognizer: 0x4664d60; state = Changed; delaysTouchesBegan = YES; view = <IOSEAGLView 0x46f6f00>; target= <(action=handlePan:, target=<XBMCController 0x46f9420>)>>
```
so the m_firstGesturePoint is not inited yet when we got the swipe action, the old code will wrong goto the previous slide when swipe left.