Provide resource filter that matches nested projects

[rherrmann]

Nested projects filters are intended to assist with multi-module maven projects or similar project structures as described here: http://jmini.github.io/blog/2017/2017-01-27_ressource-filter.html

In this context, it would help to have a filter that matches nested projects already known to the workspace so that the parent projects shows only root files and folders with projects that are not yet known to the workspace.

[rherrmann]

Commit 510727a98bf85ade93ec0caddcde7dffd1795512 adds an experimental resource filter that matches nested projects. To use the filter, open the properties dialog of the parent project, go to the Resource > Resource Filters page and add a filter with these properties:

Known limitations:

• projects nested more than one level (e.g. parent-project/child-project/grand-child-project) aren't fully recognized - the grand-child-project is filtered from the parent as soon as the child-project is known to the workspace even if the grand-child-project is not known to the workspace
• if nested projects are added or removed from the workspace, the parent project needs to be refreshed manually.

If this change proves useful, 'unit' tests for NestedProjectFilter need be added to back the current end-to-end tests in NestedProjectFilterIntegrationPDETest.