Improve constructor for TextLoader

[Zruty0]

We currently have

public TextLoader(IHostEnvironment env, Column[] columns, Action<Arguments> advancedSettings, IMultiStreamSource dataSample = null)
public TextLoader(IHostEnvironment env, Arguments args, IMultiStreamSource dataSample = null)

We need to:

1. Make the second constructor internal or private (it's used only by cmdline/dependency injection.
2. Add 'separator chars' and 'has header' as non-advanced parameters

The first change will naturally trigger a lot of changes in the code that creates the TextLoader, this is by design.

[Prsna23]

I would like to work on this issue.

[artidoro]

Will close this issue as it was taken care of in #1784.