Deprecated string defined scripts in favor of reference. Enabled gui scripts.

[IgnaciodelaTorreArias]

Resolves:
Deprecating string scripts
python-poetry-core TODO
Continuation to my PR on python-poetry/poetry

• Added tests for changed code. Updated previous Test
• Updated documentation for changed code.

As for the deprecation work changes tests toml's to reference scripts, except the ones that shouldn't of course.
Documentation updated on PR

Warning

On tests the deprecation raises a warning on test_builder_convert_entry_points that i don't know how to solve

[radoering]

I do not think that we have to deprecate anything to support GUI scripts, do we?

[IgnaciodelaTorreArias]

Well there was a TODO in code (# TODO: deprecate this in favour or reference) so i went ahead, maybe i misunderstood what that meant.
The support for GUI scripts doesn't depend on deprecation, i could separe this into 2 different PR's, if that's the right thing to do?

[radoering]

Ah, I see. In the light of #708, which deprecates most of tool.poetry.scripts anyway, I do not think it makes sense to address this TODO now. (If I was completely sure that #708 will be included in the next version, I would even say that we do not have to extend tool.poetry.scripts at all.)