- instead of showing checkboxes, we use the sl-tree-item.selection marker (blue left border) to show the multi-selection and sl-tree sl-selection-change event to set the value accordingly
- implement Et2Tree.setSubChecked(_id, _value) to allow apps to (un)check a hierarchy onclick of parent, still allowing to (un)select single children
- also change several tree methods to return the updateComplete promise to use in mail app.js instead of window.setInterval() to wait for tree loading
- Fix category tree structure
- Switch on tree multiple
probably lots of bugs still, looks like we may have to do click on tree = add / remove and not show the value after all
