Issue #11211: Fix editing custom NDE filters

This patch attempts to resolve #11211 (closed).

Currently, the code that moves the temporary "editing" filter when you're changing an NDE filter only runs for standard GEGL operations. It does not run for filters with custom dialogues like Curves. Thus, the editing filter always goes on top of the filter stack, which gives an incorrect preview of what the image will look like.

This patch moves the reorder code to gimp_filter_tool_set_config (), which always runs no matter what kind of NDE filter is being edited. It also fixes a separate problem noticed while testing this issue. Currently if you make a selection, apply a Curve filter to it, clear the selection, and then edit the existing Curve filter, it won't retain the original's filter mask. Copying the mask was handled by the same code that reordered the editing filter, so that's now fixed too.

I want to test the other tools to be sure there are no other issues. If anyone sees any problems with this change, let me know!