Dynamic adding and removing of texture layers

REVIEW: 123202

TextureLayer class was created based on parsing the .dgml content only.
With this patch, MarbleMap API has been extended to allow a dynamic
updates of texture layer (i.e. from the qml form-like interface, by
pointing to a source directory and reading/inputting some parameters,
by pointing to a geotiff and making tiles on the fly etc.). Theses
custom textures are then added to static texture list and displayed.