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
The inclusion of `(self)` was to indicate that the first object sent with the signal was `self`.