Display labels in observer's update refresh interval slider

Wx's ```wx_SL_LABELS``` macro seems to be wrongly set to 32. According to ```wxWidget```'s official ```slider.h``` [file](http://docs.wxwidgets.org/3.0/slider_8h.html#ab11c6aa70bee53b6af0f4d8b4d9a6f7d), ```wx_SL_LABELS``` should be set to  24576 instead.

Missing ```wxSL_MIN_MAX_LABELS``` and ```wx_VALUE_LABEL``` macros are also introduced in this patch.

I noticed this error because the observer application was not displaying the labels of the slider widget properly, but this error should be affecting other wx-based applications making use of the slider widget.

This is how observer's update refresh interval look like before this patch:
![slider-before](https://cloud.githubusercontent.com/assets/603610/2942563/af054392-d9b6-11e3-9476-6aa4a5d0bda6.png)

This is the result after applying the patch:
![slider-after](https://cloud.githubusercontent.com/assets/603610/2942451/9681c392-d9b4-11e3-8541-b53af3c1e4b5.png)