Refactoring

[philips77]

This PR refactors the source code of the DFU library, test app and the nRF DFU app:

1. The Pod module name of the DFU Library was changed from iOSDFULibrary to NordicDFU.

Important

This change applies only to CocoaPods. It requires changing all import iOSDFULibrary to import NordicDFU.

2. The library folder was renamed from iOSDFULibrary to Library.
3. The test app folder was renamed from Example to Test App.
4. Multiple folders inside each app were renamed to better reflect their content.
5. MacOS sample (which wasn't working anyway) was removed.
6. Unused files removed.
7. nRF Mesh app (App folder) is using local NordicDFU dependency, not from GitHub.