code improvement in \cms\data\page\Page

- order methods by their name
- documentation of methods
- implement `\wcf\system\breadcrumb\IBreadcrumbProvider` and add
`getBreadcrumb`
- rewrite `isVisible` to take `canViewPage` into account when page is
invisible or disabled as well
- implement `\wcf\data\IPermissionObject`. Therefore, default values of
`getPermission` and `checkPermissions` where removed
Merge pull request #94 from ray176/patch-1

code improvement in \cms\data\page\Page