Ignore layers that aren't tile layers

[albin-johansson]

Current map import implementations do not check the type of layers which can lead to crashes.

• Make JSON and TMX import check types of layers.
• Ideally, provide some feedback (dialog) to user that the map that was imported uses unsupported features.
• In general, avoid throwing exceptions when importing a map goes wrong. Change behaviour to return a null map_document pointer.