Make it possible to stop autoclosing when "disabled" changes

[kevinkucharczyk]

https://github.com/cibernox/ember-basic-dropdown/blob/master/addon/components/basic-dropdown.ts#L93-L105

Currently, dropdowns will automatically close when the disabled prop changes. The behaviour makes sense to me in most cases, however I do have use cases where I'd prefer to keep my dropdowns open even if they become disabled while they're open.

For example (and this will be vague) I have a dropdown that renders a list of checkboxes - when a checkbox gets checked, I run a task. While the task runs, I disable all my inputs - including the dropdown in question and the checkboxes within it - but I would like to keep the dropdown open in case my users want to check/uncheck another option right after the task finishes.