fix(cropping): use RangeSlider to fix #278 | handle `None` image

[qin-yu]

(Miss-clicked sync icon when I wanted to click master to create a new branch, so 29a47c811ec5c82e3a0fe5c41bdd4f1c694478a1 was pushed to master before I finish this PR. Let's protect master? @lorenzocerrone)

Fixed the following:

• Use RangeSlider instead of FloatRangeSlider for #278
  • Now type annotation tuple[int, int] will actually make the tuple ints, no need to convert, and
  • Therefore crop_z.value :: tuple[int, int] no longer accept e.g. crop_z.value[1] =.
• Handle None image when all images deleted

Full diff between 158bc0174a026bb2df395e09577964b3277ea760 and d1448e603795a3473bc470f1033f50adb35619ec

Difference


[qin-yu]

At least after a normal merge, it looks like I pushed to branch then they got merged together : )