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
For consistency with the rest of the package, it is better to create the item and then add it to the plotItem in separate lines. To allow the one-liner, we might add something like `PlotItem.addCrosshair`. Also, the item's bounds should probably be ignored by the ViewBox auto-range:
```
plotItem.addItem(self.crosshair, ignoreBounds=True)  # auto-range does not include this item
```
..but that creates the awkward situation where the center of the crosshair might be offscreen.