Release unpaper 7.0.0

This is the first release using Meson as build system.

Annotate 6.1 release.

Update version and NEWS file for unpaper 6.1

Remove INSTALL file.

It was not really packaged, and it was the standard automake one, there's no reason to keep it around.

Update version to 5.1.

version: make the new version number 5 instead of 0.5.

tests: make sure to clean results for the new E2 and E3 tests.

Update NEWS.

NEWS: add a date for 0.4 release.