[FIX] web: properly position invisible elements in editable listview in modals

When positioning an element in an editable list view, that element is
temporarily forced to display: block thanks to the class o_temp_visible

The rule is written in LESS thanks to:
.o_list_editable .o_temp_visible { display: block!important; }

which is ok for normal invisible form fields because it is written as
.o_form_view .o_form_invisible { display: none!important; }
which has the same priority but is defined before the o_temp_visible
rule, so the display is properly forced to block.

However, for some reason, there also is the rule:
.modal .modal-dialog .o_form_invisible { display: none!important; }
which has a higher priority than the o_temp_visible rule.
This is why appearing invisible fields in editable list view in modals
are at a wrong position.

The solution introduced by this commit is the safest: adding a new
rule to force display block!important on the selector:
.o_list_editable .o_temp_visible.o_form_invisible

addons/web/static/src/less/list_view.less

@@ -79,7 +79,7 @@
     // Grouped list views
     tbody > tr.o_group_header {
         .o-vertical-gradient(#fcfcfc, #dedede);
-        
+
         .o_group_name {
             white-space: nowrap;
         }
@@ -114,6 +114,10 @@
     .o_temp_visible {
         opacity: 0;
         display: block!important;
+
+        &.o_form_invisible {
+            display: block!important;
+        }
     }
 
     .o_list_view {