Made showPanel() and hidePanel() protected

[oskarwrobel]

Suggested merge commit message (convention)

Feature: Made Link Plugins #showPanel() and #hidePanel() methods protected. Closes ckeditor/ckeditor5#4779. Closes ckeditor/ckeditor5#4776.

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It makes possible to fully test these methods and then test how link uses them without a need to execute them.

[Reinmar]

Making something protected instead of private can't close tickets which are not precisely about making stuff protected. This is really confusing. I can see the explanation that they are now testable... but there's no "Feature" here (it's an internal change if it doesn't concern public stuff) and the description of the PR only mentions changing the visibility and nothing about #94 or #95. There must be a direct link between one and another.

You need to think as an external developer when writing these things. This might've made some sense for you but even for me, this is really confusing.

EDIT: I can see that @oleq improved the message when merging this PR. Good catch. But please, do remember to format code names (#hidePanel()). Besides, the functions are actually called _hidePanel() and _showPanel() ;)

PS, please check https://github.com/ckeditor/ckeditor5-link/issues/45.