Add onlyVisible option to ignore hidden elements

[physikerwelt]

In the default configuration even hidden nodes use space. This might result in an unintuitive layout. Space is reserved for those nodes, but no content is displayed.

According to http://js.cytoscape.org/#style/visibility elements with the property display:none do not use space and elements should not use space and not be connected.

However, the cytoscape API http://js.cytoscape.org/#ele.visible provides a 'hide' method that sets display to 'none' and a 'hidden' function that is just the oposit of the 'visible' function.

Thus a new option 'onlyVisible' was added that the user can define, if invisible elements should use space.

[maxkfranz]

If you don't want elements to be in a layout don't include those elements in the eles.layout() call.

[physikerwelt]

Thank you. For reference I had to replace

  cytoscape.use(dagre);
  cy.layout({
    name: 'dagre',
    fit:true
  }).run();

with

  cytoscape.use(dagre);
  cy.elements().filter(e => e.visible()).layout({
    name: 'dagre',
    fit:true
  }).run();