Use a single launcher

There are too many launchers packaged. It is confusing and a waste of disk space.

For instance, on GNU/Linux we have:
1. /launcher (3.7 Mo)
2. /nxdrive (3.7 Mo, the good one)
3. /nuxeo-drive-.../nxdrive (3.7 Mo)
Result: 3 same files, 7.4 Mo wasted.

On windows we have:
1. /launcher.exe (~800 ko)
2. /ndrive.exe (~800 ko)
3. /ndrivew.exe (~800 ko, the good one)
4. /nuxeo-drive-.../launcher.exe (~800 ko)
5. /nuxeo-drive-.../ndrive.exe (~800 ko)
6. /nuxeo-drive-.../ndrivew.exe (~800 ko)
Result: 6 same files, 4 Mo wasted.

And on Mac it is a mess.