This is useful when the target platform/packaging is not actually supported, e.g.
currently the code does not work when Syncthing Tray is packaged as Flatpak.
* Add include directory for testhelper library manually as using the
imported target was not possibly
* Fix visibility when using libsyncthing in syncthingwidgets
* Update changed global headers along the way
Apparently this is still problematic leading to errors like
`Qualifying with unknown namespace/class`. It seems that moving the enum to
a different header file works. The trick to use a macro to define
`enum class` ceased to work on the other hand.
Specifying a function with the namespace like
`void QtGui::FinalWizardPage::showResults()` leads to the same problem and
also needs to be avoided.
Martchus