feat(dia.Paper): add cellView show/hide API

[zbynekstara]

Description

Adds requestCellViewVisibility(cell, toggle) and isCellViewExplicitlyHidden(cell) functions:

• To request the cellView of a given cell to be shown, call requestCellViewVisibility(cell, true).
• To request the cellView of a given cell to be hidden, call requestCellViewVisibility(cell, false).
• To check whether the cellView of a given cell is currently explicitly hidden, call isCellViewExplicitlyHidden(cell).

Link visibility automatically responds to explicit element visibility:

• Any link that depends on an element (i.e. an element is its source/target, or the link is connected to a link that has an element as its ultimate source/target) gets hidden when the end element in question is explicitly hidden.
• Similarly, if an explicitly hidden element was the only thing that previously prevented any links from being shown, those links get shown when the element in question is explicitly shown.

Note that requestCellViewVisibility(cell, toggle) triggers an asynchronous request - the change happens on next animation frame. If the change is needed immediately, use requestAnimationFrame() or trigger paper update manually via paper.checkViewport(); paper.updateViews();.

Motivation and Context

Allows specific models to be set which should always be hidden no matter what the viewport paper option says.