ui: add form for editing tags

ui: add form for editing tags

This is a wrapper container for holding a list of (editable) tags
intended to be used in the lxc/qemu status toolbar

To add a new tag, we reuse the 'pveTag' class, but overwrite some of
its behaviour and css classes so that it properly adds tags

Also handles the drag/drop feature for the tags in the list

When done with editing (by clicking the checkmark), sends a 'change'
event with the new tags

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>