[FIX] web_editor: make save/discard btns visible when too many options

The editor bar is sometimes filled with too many options, and these
hide the save and discard buttons on small screens. To solve the
problem, the description option (which take a lot of space) has now
a max-width and the save and discard buttons are always on top.

addons/web_editor/static/src/less/web_editor.ui.less

@@ -12,10 +12,13 @@
 
     form.navbar-form {
         height: 100%;
+        z-index: 1060;
         margin: 0;
         padding: 0;
         .o-position-absolute(@right: -@o-we-sidebar-width);
         .o-transition(right, 0.4s, @o-we-md-ease);
+        border-left: 1px solid @o-we-color-divider;
+        background-color: inherit;
 
         .btn-group {
             height: 100%;
@@ -703,6 +706,7 @@ html[lang] > body.editor_enable{
 
     .popover-content {
         height: @odoo-navbar-height;
+        white-space: nowrap;
 
         .btn {
             height: @odoo-navbar-height;
@@ -717,6 +721,11 @@ html[lang] > body.editor_enable{
                 border-top: 2px solid @o-we-color-text-normal;
                 color: @o-we-color-text-normal;
             }
+
+            .o_image_alt {
+                .o-text-overflow(inline-block);
+                max-width: 100px;
+            }
         }
         .btn.active {
             border-top: 2px solid white;