Move LImageOverlay url prop from mixin to component

[mandryllo]

This PR movesLImageOverlay url prop from mixin to a component so it doesn't end up in the components that inherit its mixin (like LVideoOverlay #683 or LSVGOverlay #684 )

If this PR is approved it should be merged before these two mentioned above because they depend on it.

[mikeu]

Hi @mandryllo , thank you for these contributions! At first glance they look good to me, and I should have time to review them in more detail and merge them soon. In the meantime, I am curious if you believe this particular PR is still relevant, since it looks to me as if the changes it provides are already merged into the branches of your #683 and #684 PRs, so will be included once either or both of them are merged. Do you agree, or did I miss something about this one?

[mandryllo]

@mikeu Yes, you are right, I opened this PR after those two but decided to merge it because I wanted to update their docs also 👍