highgui: Improved Trackbar

I've made 3 (small) improvements/fixes to the Trackbar in Highgui:

1. Added new API function for changing the trackbar's max count after creation. Useful for dynamically changing the range.

2. Using the correct, Windows TBBUTTONINFO, instead of the custom one. This fixes incorrect trackbar width due to incorrect struct "version."

3. Replaced the deprecated CreateToolbarEx, which avoids a warning (on newer VS) and a library dependency.