Rename Enginio::ErrorTypes to ErrorType.

[ChangeLog][API] Enginio::ErrorTypes was renamed to Enginio::ErrorType

Change-Id: I03e3c724c2fc4b28aba2cd58b1776a005e5c077e
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Rename Enginio::LastRole to Enginio::CustomPropertyRole

Fix small mistakes related to Enginio::Role documentation

Change-Id: Ia4241e750f684b80cdf0984d44939c53f08e19df
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Make QML headers private

Change-Id: I7469abf471bc1ff2fe9eb8144a8a90ee3bc72f0d
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>