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
This should be a subclass of `GraphicsObject` (or similar). That will allow you to make the lines and text children of this item, and thereby allow you to add/remove the item as a whole from a scene. This should allow to remove the `addItems` and `removeItems` methods. It would also make things like `crosshair.hide()` work as expected.