Resizable handle flag hideHandlesWhenNotResizable added, extend example

[timmaffett]

This PR adds a new flag hideHandlesWhenNotResizable which allows user to request that the handles be automatically hidden when not resizable.

This PR also extends the example to include a switch to set hideHandlesWhenNotResizable`, as well as an additional switch to add a second ImageBox() - the model and imagebox have been extended to allow this, as well as generalizing ImageBox() to not be as hard coded to the specific model rect/flip flag and asset image name.

It also adjusts the const declarations so that it does not have analysis issues (which may be because I am on master branch of flutter).

[timmaffett]

I will also note that the example really is not well architected for having multiple boxes, but I do think it is important to have this because I would imagine that most users of this library would be using it on multiple items, unless of course they only used it on the currently selected item within their architecture.
I think a more generalized example with better architecture for multiple items would be useful for users to illustrate how to do this properly.

[BirjuVachhani]

@timmaffett PR build and quality checks are failing. Can you check if everything is formatted properly and the PR is compatible with Flutter stable channel?

[timmaffett]

I have corrected the dart format issue. It should now pass the workflow. Sorry about that!

[timmaffett]

..and I have reverted the const changes that my master channel dart analyze was suggesting - I didn't realize that master<->stable was so different about this!

[BirjuVachhani]

Sorry for all the notifications & inconvenience. I was trying to fix workflows for building demo links for PRs but seems like Github doesn't allow using secrets for PR workflows so I can't deploy.

[BirjuVachhani]

LGTM

[BirjuVachhani]

@timmaffett Thank you for your contributions! 🎉