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.
- Add enums to qgis.h instead of qgscoreenums, so that they belong to
a Qgis namespace
- Split up the various symbol headers into multiple files so that we
can fine-tune their inclusion and forward declare more readily, speeding
up recompilation
- Move QgsSymbol enums to qgis
there is an issue in SIP when mixing non-scoped and scoped enums in the same class
see https://www.riverbankcomputing.com/hg/sip/rev/ccc4eda868de
and if switching all to scoped enum, there is a conversion issue in slot
for MessageLevel (whhen connection QgsMessageLog::messageReceived)
