Added AbstractApplication::isActiveApplication()

Added AbstractApplication::isActiveApplication()

If you're developing an own application, simply add a protected variable "$abbreviation" and assign the application's abbreviation (e.g. "wbb"). Furthermore the package id is automatically set and can be accessed through $this->packageID.

In case you're setting the active page menu item and adding a "top" breadcrumb for your application, please do so if $this->isActiveApplication() returns true. If you fail to check this, multiple applications will stack their breadcrumbs and the active page menu item would no longer be deterministic.