class
This avoids an ugly include of qgsvectorlayerexporter.h in
qgsproviderregistry.h/qgsprovidermetadata.h, which in turn
triggers an inclusion of qgsvectorlayer.h and a bunch of
other heavy dependencies.
are to be clipped from the project, instead of just tracking this
by the presence of any checked layers
Avoids inconsistencies between the layers which are visibly clipped
on the map vs the options which are set in the GUI.
