Enable Felix SCR BndTools plugin

The Felix SCR BndTools plugin is currently disabled in Sling Parent:

[...]
                            <Bundle-RequiredExecutionEnvironment>${sling.bree}</Bundle-RequiredExecutionEnvironment>
                            <!-- Disable the new ScrPlugin for now -->
                            <_plugin>org.apache.felix.bundleplugin.BlueprintPlugin,aQute.lib.spring.SpringXMLType</_plugin>
                        </instructions>
[...]

I tried several configurations to restore a working build of org.apache.sling.event with maven-scr-plugin and maven-bundle-plugin including testing (missing SCR descriptors) to no avail and finally removed maven-scr-plugin and enabled org.apache.felix.scrplugin.bnd.SCRDescriptorBndPlugin in r1787036 .

Using Felix SCR BndTools plugin is a quick fix obviously before switching to official OSGi annotations.

We should enable Felix SCR BndTools plugin to not block modules upgrading to Sling Parent >= 30 (without reconfiguring Maven Bundle Plugin in their own POMs or switching to official OSGi annotations).