Abstract scene parser class for all scene parsers

Base parser class that has info and errors logs methods.
It also has a parserStatus property to reflect the various
states the scene parser can be in.

Change-Id: Ic314ba5bd6075492c0faf5b9a5196affb7802158
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Initialized m_sceneChild in scene to Q_NULLPTR.

Change-Id: I30122e6bfead056484d3fd896f229c624510a8b1
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Corrected gltfparser KEY_SAMPLERS value. Textures are now visible.

Change-Id: I72aa71e1173ae30a2cac38d20cb3ff2bb5d09ac6
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>