Extension properties should be lazily configured

[netvl]

Right now, the extension object directly configures default values of its properties in the constructor:

        dataDir = project.objects.property(File)
        dataDir.set(new File(project.buildDir, 'scoverage'))

This is, however, a problem, because project.buildDir may be changed by other plugins. This results in the plugin producing output in unexpected places if the plugins are applied in a certain order.

Instead, the lazy properties API has the "convention" mechanism accepting providers, which is supposed to be be used for default values:

    dataDir = project.objects.property(File)
        .convention(project.layout.buildDirectory.dir('scoverage'))  // dynamic default values should be providers

    highlighting = project.objects.property(Boolean)
        .convention(true)  // static default values can be immediate