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
Probably not necessary to include any of the three `_Label` classes here because they are used internally by `InfiniteLine`, `LinearRegionItem`, etc. They should only be here if you expect users to manually instantiate them at some point..