text: Title: Add intensity_median to regionprops
  Body: ### Description:
  
  We have `intensity_min`, `intensity_mean` and `intensity_max` in `regionprops` already:
  
  https://github.com/scikit-image/scikit-image/blob/5305ae2ae1fa5d8c01a45e77bb0c44cbabfe1102/skimage/measure/_regionprops.py#L573-L585
  
  What about adding `intensity_median` as well, backed by `np.median` in a similar way as the other props?
  
  I can open a pull request if there are no general objections against this idea.