ComboBox is unuseable - PosRelative in PointEditor

[tznind]

ComboBox is not working at all for editing pos relative

Issues are

• ComboBox tab completion fails under some scenarios Terminal.Gui#3685
• ComboBox Cursor.Down should expand dropdown Terminal.Gui#3684

Also a visibility related issue where Side becomes invisible

It is currently impossible to set Pos.Left something then switch to Pos.Right

Consider changing to simple text box with suggest append autocomplete.