[Projects] Moved GetReferencedItems method to AbstractDProject for re-implementing this method in both normal and dub projects
[Projects/Dub] Renamed DzbProject.GlobalBuildSettings to CommonBuildSettings
[Projects] Properly use OnBeginLoad/OnEndLoad events.
[Projects/Dub] Make DubBuildSettings et al cloneable
[Projects/Dub] Use a string constant as default dub configuration id
[Projects/Dub] Handle sub packages properly: Add them as separate projects and have them as reference
[Projects/Dub] Copy DubProjectConfiguration.BuildSettings as well for consistent cfg copying
[Projects/Dub] Corrected dub config parsing: Only clear all previous configs if there's only the default config in a dub project object