[ADD] web_tree_column_width_remember: Remember the tree view columns' widths

Remember the tree columns' widths across sessions, and after filtering, grouping, or reordering.

web_remember_tree_column_width/static/src/scss/main.scss

@@ -0,0 +1,8 @@
+th.o_column_sortable {
+    max-width: none !important;
+    min-width: 0 !important;
+}
+
+.o_optional_columns_dropdown_toggle {
+    height: 25px;
+}