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
There seems to be a lot of copied code between InfiniteLine and LinearRegionItem. Duplicates should be removed wherever possible. It is probably not necessary for this class to keep track of `self.visibleLabels`, `self,nDigits`, etc--InfiniteLineLabel should probably be the only place where these appear?