Make image optional when initializing PV slice

When making the standalone PV slicer, I found that it would be handy to be able to initialize a PVSlicer widget with no pre-existing data.
@ChrisBeaumont - I was digging into this code, and was wondering if there is a reason why you transform the position of the event on the fly, rather than using ``imshow`` with the ``extent`` keyword. In the case of the PV slice I think we always expect the pixels to be evenly space, so we don't technically need to keep track of the full array of ``x`` and ``y``, right? In fact, the PVSliceWidget could even just use WCSAxes which gives the full transformation by default, making this kind of thing trivial. Shall I try and switch to using WCSAxes instead, for ``PVSliceWidget``?