Check disabled on layer remove, fix re-adding of controls overlay

[ahmadayubi]

2021-02-25.09-57-13.mp4

Closes #310 and closes #318

[Malvoz]

This is great @ahmadayubi, though it seems to me that the Remove Layer button shouldn't be clickable if fieldset is disabled per https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#attr-fe-disabled (quoting the directly relevant parts):

A form control is disabled if any of the following conditions are met:

2. The element is a descendant of a fieldset element whose disabled attribute is specified

A form control that is disabled must prevent any click events that are queued on the user interaction task source from being dispatched on the element.

But, if it's important that layers can be removed even if a layer item (<fieldset> as currently implemented) is disabled then perhaps that should be a consideration (for later) when refactoring the layer items' anatomy in #263.

[Malvoz]

Maybe I'm being a bit too strict about conforming to the HTML spec's behavior in this matter.