[Qbs] Same precompiled headers for multiple CppApplications

[Mitja Schmakeit]

Hi,

is it possible to have a single

             Group {
                 name: "Precompiled Headers"
                 files: ["precompiled-header.h"]
                 fileTags: ["cpp_pch_src"]
             }

where the precompiled-header.h is only compiled once for multiple 
CppApplication-products? This would be nice for our unit test applications.

Mitja