Use config file for examples

The examples will try to use a file named
EnginioExamples.conf
and if it doesn't find any, it uses the system config (.config/com.digia/EnginioExamples.conf)

The config must look like this:

[image-gallery]
backendId=@ByteArray(abcdef...)
backendSecret=@ByteArray(Aabcdef...)

[todo]
backendId=@ByteArray(123abcdef...)
backendSecret=@ByteArray(1234abcdef...)

The approach can later be extended to even create a new backend for the example.