I added Rok's suggestion to the specification.
About its implementation, I agree with Steren's idea: let's use a button
for matching original aspect ratio keeping the larger dimension and
modifying the smaller.
That would be the very useful to solve a very common situation: You
imported an image, then you want to crop it (for instance, you have a
sponsor logo with a lot of transparent space around and you want to get
rid of that empty space and leave the logo only).
Thanks for the feedback.
p.s.: Steren, It'd be great if you could mentor a team of students to
work on this feature. It can be made in different stages and it doesn't
look too complicated to be finished in a short term.