USD Import: mask multiple prim paths.

This patch addresses bug #102594.

These changes enhance the prim_path_mask USD import
option to allow masking on multiple paths.

- Changed the prim_path_mask import property to
a string of arbitrary size.

- Multiple paths may be specified in a list delimited
by spaces, commas or semicolons.

- Now using the pxr::UsdStagePopulationMask class to apply
the masking when opening the stage, which is more efficient
and which preserves the full prim paths.

@@ -623,0 +620,4 @@
+      0,
+      "Path Mask",
+      "Import only the primitive at the given path and its descendents.  "
+      "Multiple paths may be specified in a list delimited by spaces, commas or somicolons");

@@ -376,6 +390,8 @@ static void import_endjob(void *customdata)
       break;
   }
 
+  MEM_freeN(data->params.prim_path_mask);