silx.gui.plot.items: Added item visible bounds feature to PlotWidget items

[t20100]

This PR adds method Item.getVisibleBoundsExtent to PlotWidget items to retrieve the visible extent of the item bounding box in the plot area.
It also adds a private API to enable tracking changes of this visible extent through the Item._sigVisibleBoundsExtentChanged signal.
This signal is only triggered when enabling "visible extent tracking" through Item._setVisibleBoundsExtentTracking and which can be retrieved throughItem._isVisibleBoundsExtentTracking.

[vallsv]

Be careful that Tuple concept is like C, fixed number of typed elements. Tuple[float] is a tuple of a single float. Here you want Tuple[float,float,float,float].

Maybe it would be better to rename getVisibleExtent with bound something?

[t20100]

Renamed visibleExtent to visibleBounds and fixed getVisibleBounds typing.