Proposition of extra functionnalities to existing files, new features and dedicated examples

add a remapSlider method used by some GraphicsItems in the functions file, add some extra functionalities to the InfiniteLine and LinearRegionItem items, add a InfiniteLineLabel item, a LinearRegionItemLable item, a CrosshairLabel, a new CounterSlider widget add four dedicated example (on per new item) and modify the __init__ file in the main directory to import the four new files
It was required because I needed to get the Range of the plotItem in the class to be able to set the location of the items within the PlotItem depeding on the drag of the line. But it seems unnecessary since I can directly get the ViewBox. Will be change soon.