Issue #12791: Lock filters when editing
This patch attempts to resolve issue #12791 (closed).
When editing a filter, the NDE UI is set to insensitive so they can't be deleted. However, closing and reopening the popover re-enables the options.
This patch adds a new "temporary" boolean to GimpDrawableFilter. It is only set to TRUE if the filter is a temporary "editing" filter. When we reopen the NDE UI, this checks if the filter stack contains a temporary filter, and if so, once again locks the options.
@brunolopesdsilv Hey! Let me know when it's okay to merge this.