[FIX] web: hide list readonly checkbox when entering edition

Forward-port of https://github.com/odoo/enterprise/commit/8a55b3f9f7a7524583effb8198c03ebf695131bd

There are multiple problems with the editable view list. It should be
solved once and for all in the work-in-progress master.

This commit fixes a bug with readonly checkboxes. When a field is
readonly, the table cell gets the "o_readonly" class which changes
its style and behavior. When entering edition, the cells are hidden
except the ones with this "o_readonly" class so that their style are
not affected. The "o_readonly" cell content however was hidden thanks
to a "color: transparent;" rule to let the form readonly field show
the value. The problem is that checkboxes... are not hidden with this
"color: transparent;" rule. Also, the "o_readonly" class is not toggled
during edition while the readonly status could be... so the right fix
would be to get rid of this "o_readonly" class and let the inline form
handle the behavior, but this would affect style and possibly break
unknown behavior.

This fix adds rules to handle the checkbox case correctly.

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

@@ -135,6 +135,10 @@
             &.o_readonly {
                 visibility: visible;
                 color: transparent;
+
+                > .o_checkbox {
+                    visibility: hidden;
+                }
             }
         }
     }
@@ -155,7 +159,7 @@
                 max-width: 100%; // Fix a chrome bug where badge would not break on multiple lines
             }
 
-            &.o_readonly {
+            &.o_readonly:not(.o_checkbox) {
                 background-color: transparent;
                 padding: 5px;
             }