Adding support for list property types

[bjorn]

Implementation so far:

• Added 'list' as option to the Add Property dialog, for adding a QVariantList.
• Adjusted the CustomPropertiesHelper to create sub-properties that display the values in the list (maybe should rather be done by the VariantPropertyManager).
  • It is not done by VariantPropertyManager because it couldn't delegate class member creation back to the CustomPropertiesHelper. If VariantPropertyManager should handle lists, then it would need to handle classes as well.
• Added support for saving and loading list properties to TMX and JSON formats.

• Added a tool button with menu that allows to add new list items.

ToDo:

• [x] Ability to add items to a list
• [ ] Ability to remove items from a list
• [x] Editing list items
• [ ] Reordering list items

Resolves #1493

[bjorn]

Urgh, turned out the "Ability to add items to a list" is only mostly working. When just pressing the "+ Add" button instead of using the menu, it is not possible to add items of the same custom enum or class type. This is because the values are at that point converted to their "display value", which means a class is just a map and an enum is just a number, and only the CustomPropertiesHelper knows the associated type.