Adds the ability to set a maximum height of the legend

[Nathan219]

This PR adds the ability to set a maximum height for the legend. As far as I know, there is no way to currently do this, so (in combination with the negative anchor bug), causes large legends to completely block the graph. Without this, it's almost impossible to make sure the legend has enough extra room to fit.

Please help me add this functionality

Before opening a pull request, developer should:

• git rebase their local branch off the latest master,
• make sure to not git add the dist/ folder (the dist/ is updated only on version bumps),
• make sure to commit changes to the package-lock.json file (if any),
• write an overview of what the PR attempts to do,
• select the Allow edits from maintainers option (see this article for more details).

Note that it is forbidden to force push (i.e. git push -f) to remote branches associated with opened pull requests. Force pushes make it hard for maintainers to keep track of updates. Therefore, if required, please git merge master into your PR branch instead of git rebase master.

[Nathan219]

@n-riesco
@antoinerg
@etpinard

[etpinard]

@Nathan219 Thanks for the PR!

Could you point us to which issue you're trying to solve?

[etpinard]

This might conflict with the upgoing plotly#3024