Support nested targets

[laeubi]

PDE supports nested targets now, tycho should support this as well.

[mickaelistria]

Please add a note about it to release notes; ideally referencing Eclipse PDE help about how this works for .target file in PDE and telling Tycho now mimics it.

[laeubi]

See https://github.com/eclipse/tycho/blob/master/RELEASE_NOTES.md#support-for-nested-targets

[mickaelistria]

See https://github.com/eclipse/tycho/blob/master/RELEASE_NOTES.md#support-for-nested-targets

Thanks, I missed it.
Ideally, the nested platforms would be described in https://help.eclipse.org/latest/index.jsp?topic=%2Forg.eclipse.pde.doc.user%2Fguide%2Ftools%2Ftarget_shared%2Fedit_target_wizard.htm and/or some other docs of PDE and Tycho would just reference it.