Turn GIS implementation into a plugin

Under terms of this task we'll do the following:

• Move the following source files from /sql/ to /plugins/type_geom/:
  • gcalc_slice_scan.h
  • gcalc_slice_scan.cc
  • gcalc_tools.h
  • gcalc_tools.cc
  • item_geofunc.h
  • item_geofunc.cc
  • spatial.h
  • spatial.cc
  • sql_type_geom.h
  • sql_type_geom.cc
• Move test files gis*.result and gis*.test from /mysql-test/ to /plugins/type_geom/mysql-test/:
• Wrap type_handler_geometry (and its sub-types) into MariaDB_DATA_TYPE_PLUGIN instances
• Wrap all SQL functions defined in func_array_geom[] in item_gefunc.cc into MariaDB_FUNCTION_PLUGIN instances