Refactor pager class

[harro]

While preparing the 2.0.0 release I noticed some inconsistent Pager behaviour.
I subsequently ran into trouble trying to localise the issue by adding tests...
So I spent the weekend refactoring the Pager class to make unit testing easier. I added many more tests, then found and resolved several issues.

Issue(s) resolved:
SetLines was being ignored
Percent was conditionally displayed
Only every 2nd 'enter' key was being registered by tty.read
Prompt string did not truncate on narrow terminals
Added support for MS Windows

Apologies for the lengthy CL - I combined it all together so we can proceed with system/regression test as part of this CL review.

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[harro]

Updated list of issues resolved ...

1. First line of output was not being displayed (off by one error).
2. SetLines was being ignored. Page size was fixed to terminal size.
3. Percent was conditionally displayed. It now displays when paging additional input.
4. Only every 2nd 'enter' key was registered by tty.read, switched to using 'n' key instead.
5. Prompt string did not truncate on narrow terminals
6. Added paging support for MS Windows terminals

[harro]

• Pager no longer exits automatically at end of buffer - This is the preferred behavior
• Fixes day-one issue of non displaying empty lines

[harro]

Oh, and returned the conditional display of the % progress, as this is the preferred behavior for streamed input

[harro]

Also correctly handles files smaller than the terminal