Add ECMOptionalPackageProperties module

This is already used in Okular to make optional required by default unless the user explicitely opt-out to make the dependency optional.

This guarantee that by default distributions will package the dependency unless they explicitely have a reasons to disable a feature.

Usage:

• libkdepim: pim/libkdepim!23
• kjobwidgets: kjobwidgets!56
• okular: graphics/okular!1004