Improve product branding to support arbitrary macOS Info.plist attributes

[sratz]

The https://github.com/eclipse-equinox/p2/blob/cc3868542ee18f101b56a5a02916b148ad13819f/bundles/org.eclipse.equinox.p2.publisher.eclipse/src/org/eclipse/equinox/internal/p2/publisher/eclipse/BrandingIron.java is currently quite limited when it comes to customizing products for macOS.

E.g., for eclipse-platform/eclipse.platform.swt#2627 it is currently necessary to modify the Info.plist file manually as part of the build process (e.g. via maven-antrun-plugin or https://github.com/buchen/fix-info-plist-maven-plugin).

It would be nice if the branding on macOS could be made more dynamic, by allowing arbitrary attributes for the Info.plist to be configured in the product configuration.

[sratz]

Idea from the linked issue:

• Current options:

  <launcher name="Foo">
     <macosx icon="icons/AppIcon.icns"/>
  </launcher>

• Maybe add a more general syntax here, something like

  <launcher name="Foo">
     <macosx>
       <icon>iconsAppIcon.icns</icon>
       <additionalResources>resources</additionalResources> <!-- local folder whichs is copied to the Bundle under Resources during publishing -->
       <additionalPlistConfigurations>
          <!-- ... -->
       </additionalPlistConfigurations>
     </macosx>
  </launcher>