Remove unnecessary tabindex (and native outline) from dropdowns

[mmotyczynska]

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After adding tabindex to the dropdown panel view, some of the dropdowns may now have unnecessary tabindex set as well. One example is the Special Characters dropdown view, where tabindex was added some time ago to prevent the dropdown from closing when a user clicked between the buttons. But now we have tabindex in the dropdown panel view, so in this case there is no need to keep it: https://github.com/ckeditor/ckeditor5/blob/e8bc4cd96381a5add990822501d6d716fc72a2ab/packages/ckeditor5-special-characters/src/ui/specialcharactersview.ts#L94-L98

The side effect of the tabindex is that when the user clicks the area of the panel view or the scrollbar (if exists) and then the user presses any key on the keyboard, a native ugly outline is displayed (if not turned of in CSS).

https://github.com/ckeditor/ckeditor5/assets/95343030/34b51b96-9be4-41ef-b9e1-2fe965da9b3a

A second example is the Template feature. There may be other similar cases, so the scope of this ticket would be to find all such cases and remove the unnecessary tabindex.

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