#5462 Added ability to preselect active page upon loading the project

[padcom]

This PR introduces a new configuration option for the PageManager:

pageManager: {
  activePage: 'initial-page-id-goes-here',
  pages: [ /*...*/ ],
},

It is useful when you want to start editing a specific page rather than the one with { type: 'main' }, e.g. when you use GrapesJS as the editor only and the page being edited is dictated by an external source.

To test it, add the following to your configuration:

        pageManager: {
          activePage: 'test-page-2',
          pages: [
            { id: 'test-page-1', type: 'main', component: { content: '<h1>Test page 1</h1>' } },
            { id: 'test-page-2', component: { content: '<h1>Test page 2</h1>' } },
          ]
        }

which will cause the second page to be active immediately rather than the first one, with { type: 'main' }

[artf]

@padcom what do you think about following my suggestion here?

I'd say to simply try to find the selected page in onLoad (no need for getPreselectedPage method) as the project could be loaded from a remote storage