[gimp] Issue #5179: fix "Add layer masks" and improve "Edit layer mask".

[Jehan <jehanp src gnome org>]

commit 080ef8a0d88ad21aebb9a629afb395a5b911b562
Author: Jehan <jehan girinstud io>
Date:   Tue Jun 9 11:54:16 2020 +0200

    Issue #5179: fix "Add layer masks" and improve "Edit layer mask".
    
    The `mask` test was broken as it is not multi-layer aware (also it
    should be negated).
    
    The "layers-mask-edit" test was actually already fine because current
    implementation only allow mask editing when single-selected. Still
    improve the test to use multi-layer variables.
    
    Thanks to Cyril Richard for the original patch proposition from which I
    derivated the present one.

 app/actions/layers-actions.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/app/actions/layers-actions.c b/app/actions/layers-actions.c
index 275803ea9e..d655f80d77 100644
--- a/app/actions/layers-actions.c
+++ b/app/actions/layers-actions.c
@@ -1076,14 +1076,14 @@ layers_actions_update (GimpActionGroup *group,
   SET_SENSITIVE ("layers-composite-mode-clip-to-layer",    n_layers && cm_mutable);
   SET_SENSITIVE ("layers-composite-mode-intersection",     n_layers && cm_mutable);
 
-  SET_SENSITIVE ("layers-mask-add",             n_layers > 0 && !fs && !ac && mask);
+  SET_SENSITIVE ("layers-mask-add",             n_layers > 0 && !fs && !ac && have_no_masks);
   SET_SENSITIVE ("layers-mask-add-button",      n_layers > 0 && !fs && !ac);
   SET_SENSITIVE ("layers-mask-add-last-values", n_layers > 0 && !fs && !ac && have_no_masks);
 
   SET_SENSITIVE ("layers-mask-apply",  have_writable && !fs && !ac && have_masks && have_no_groups);
   SET_SENSITIVE ("layers-mask-delete", n_layers > 0 && !fs && !ac && have_masks);
 
-  SET_SENSITIVE ("layers-mask-edit",    layer && !fs && !ac && mask);
+  SET_SENSITIVE ("layers-mask-edit",    n_layers == 1 && !fs && !ac && have_masks);
   SET_SENSITIVE ("layers-mask-show",    n_layers > 0 && !fs && !ac && have_masks);
   SET_SENSITIVE ("layers-mask-disable", n_layers > 0 && !fs && !ac && have_masks);