Make jsonform properties button more visible / clearer as button

[santilland]

Re-enabling the properties button ("disable_properties": false) is a great way to hide complexity of optional properties to the user.
The problem i see is that the button itself does not look like a button that invites to be clicked, it looks like a decoration icon to the title.

Can we make this look more like a button? maybe give it classical rounded outline? or any other way you think would fit?

[silvester-pari]

We can make it more prominent, but in nested schemas it might be a bit obnoxious since this button is present at every level (for example objects in arrays all have this button). Will need to check how to solve that, e.g. by making only the topmost more prominent and all the others not.