Changed NULL to nullptr.
Merge pull request #634 from Morwenn/patch-1

Changed NULL to nullptr.
Rename CommandLine to arguments and allow to add some programmatically.
Allow to throw an exception instead of aborting on assertion errors.

This is necessary to use Solarus as a library.
Include Solarus headers with the explicit Solarus directory.
Remove dead code.
Clear the resource list when Solarus stops.
Remove useless enum value.
Add functions to get resource type name and by name.
Allow to load resource list from any source.
Resource lists can now be loaded from arbitrary files.
Add functions to get/set a resource element description.
Fix static initialization order crash.
Show an error message if project_db.dat does not exist.
Fix typos in comments.
Use an std::map to map Lua names to enum values.
Allow to remove/rename resources dynamically (not from Lua yet). #630
Check that the new id is not used when renaming a resource.
Addd export feature to the quest resource list for symmetry.
Make the Color class independent of pixel formats and video initialization.

Color instances are now pure values and the code is simplified.