of new memory layers from QgsFields/QgsCoordinateReferenceSystem/etc
Since memory provider cannot use QgsVectorLayerImport we need
another centeralized function for creating new memory layers
without the need to manually create the uri string.
This commit introduces basic support for non-library based
data providers, and moves the memory provider from a library
based provider to a core provider.
The memory provider is used so frequently throughout QGIS that
it has become integral to QGIS functionality and must be
available wherever the QGIS core library is used.
The changes here (while not exposed yet to Python) could potentially
be built on to allow creation of pure Python data providers.
