[LVGL] Support groups

[fietser28]

Please add support for groups (needed for keys and encoders):

• Have a tab with the list of groups (like Global vars). The list of objects assigned to a group should be orderable (like a struct or an enum), maybe per screen?
• Add a group property to objects: Select the groups the object belongs to (multiple selections possible?)
• On screen load event assigns objects to the selected groups in the right order.