Add support for generating a Make compatible depfile

[dcbaker]

This allows fypp to communicate to build systems like Make and Ninja (including through build system generators like CMake and Meson) files that need to be implicitly included in the build graph. This is important for getting correct incremental rebuilds when such a file is changed.

[dcbaker]

Updated with the requested changes. I've also updated the test case, and added a couple of more cases, including one to explicitly test the escaping behavior.

[dcbaker]

I've rebased this across the recent code reorganization

[aradi]

OK, perfect (and sorry for the long delay times). I think everything is fine so far, and the PR should be merged.

I have one only API design question: What is the advantage of not writing the depfile already during the process_file() call, but requiring a separate write_dependencies() call. At latter, you have to pass the name of the depfile again, which had been already passed via the comman line option --depfile.

At the first glance, it seems more logical to me, to write the depfile during the process_file() call and not having the extra write_dependencies() method. Any views on this?