Extract toggle visibility of a SystemTagsInfoView to its own view

The SystemTagsInfoViewToggleView is a basic view that renders a label
that, when clicked, toggles the visibility of an associated
SystemTagsInfoView.

In order to keep the view parent agnostic its attachment and detachment
to/from the MainfFileInfoView is done in the FilesPlugin.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>