Patch introducing shapestore registry

[Jan-Oliver Wagner]

hi,

attached is a patch that introduces a simple shapestore
registry.

The rationale is to allow the load routine to call
a ShapeStore class which was registered for a shapetype.

This allows to load e.g. a MapInfo layer (I tested
this successfully with a modified ogr extension).

The registry might have to be improved further, but
as a first instance, it works.
OK to commit?

	Jan
-- 
Jan-Oliver Wagner               http://intevation.de/~jan/
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