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
Strange. These svg files should be in pyqtgraph/pixmaps. But I had a problem with these files (I was not able to use them properly). If I remember well, I add few comments in the header of the CounterSlider.py source code