Remove PlayerAction type handlers

[eviltak]

The changes in MovingBlocks/Terasology#3535 have made the existing type handlers of PlayerAction and its implementations obsolete. This PR removes those TypeHandlers since their functionality is already present in the new serialization system.

This does lead to a slight change in format of their serialization, however. The type of all PlayerActions must be specified in a class field, not a type field as was before. Moreover, when being used outside the Dialogs module, the type specified must contain the Dialogs module name in its URI.

As an example, a usage of ChangeDialogAction in the LightAndShadow module would change from

{
    "type": "ChangeDialogAction",
    "prefab": "somePrefab"
}

to

{
    "class": "Dialogs:ChangeDialogAction",
    "prefab": "somePrefab"
}

A PlayerActionSerializationTest test class has also been added containing tests asserting the format in which PlayerActions are serialized.

Should only be merged after the engine PR has been merged.

Resolves #8.