Implement dropdown component

[oleq]

Drop–downs are crucial for first features like formatting or styles and must be implemented as soon as possible.

Note: There's an ongoing discussion in design repository about UX/UI details ckeditor/ckeditor5-design#101.

An umbrella ticket is ckeditor/ckeditor5#171.

[oleq]

Main feature branch in ckeditor5 repository https://github.com/ckeditor/ckeditor5/tree/t/ckeditor5-ui-default/12

Others:

• https://github.com/ckeditor/ckeditor5-ui/tree/t/ckeditor5-ui-default/12
• https://github.com/ckeditor/ckeditor5-creator-classic/tree/t/ckeditor5-ui-default/12
• https://github.com/ckeditor/ckeditor5-theme-lark/tree/t/ckeditor5-ui-default/12