List: additionalScrollHeight

[isidorn]

This PR adds the additionalScrollHeight option to the list and propagets it through the tree classes. This option allows the list clients to specify additional scrollHeight. I decided to make this a number, not a boolean since clients are in charge of the height of each element.

Here the explorer also starts using this options, specifying twice the element height.
The motivation for this is to tackle #1043 so it would be easier for users to create top level elements in the full explorer by scrolling all the way down.

I tried it out and I actually like how it feels.
Let me know what you think and thanks for the review.

[joaomoreno]

Not too happy about this either in code nor in the product. Here are some alternative ideas to tackle #1043:

• Add additional context menu actions on files: New Sibling File, etc;
• Add a Parent Folder submenu to the file context menu, containing all actions of its parent folder;
• Tell users to right click on the scrollbar, it actually behaves as if you're right clicking the root. 😆

[bpasero]

My 2 cents: we have discussed all those alternatives in length and they are not nice. I personally found this proposed solution to be one of the best so far and would go for it.

Please do not add a new menu entry. And please do not force everyone to have a root node.

[isidorn]

@bpasero @joaomoreno and me just discussed this offline.
And decided to go with this PR and this approach. However we will decrease the empty space to be one element height, and not two.
Submlime does something similar

[isidorn]

After we selfhost on this for a couple of weeks we can revert it if we hate it, or we can tune the space at the bottom.

[bpasero]

Cool!