* Add new SKIP macro for skipping tests at runtime

This adds a new `SKIP` macro for dynamically skipping tests at runtime.
The "skipped" status of a test case is treated as a first-class citizen,
like "succeeded" or "failed", and is reported with a new color on the
console.

* Don't show "skipped assertions" in console/compact reporters

Also extend skip tests to cover a few more use cases.

* Return exit code 4 if all test cases are skipped

* Use LightGrey for the skip colour

This isn't great, but is better than the deep blue that was borderline
invisible on dark backgrounds. The fix is to redo the colouring
a bit, including introducing light-blue that is actually visible.

* Add support for explicit skips in all reporters

* --allow-running-no-tests also allows all tests to be skipped

* Add docs for SKIP macro, deprecate IEventListener::skipTest

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

Apart from cleaning up the code, this change also improves the
compilation time of `UsageTests/Matchers.tests.cpp` by about 2%.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

Apart from being clearer, it also improves the overall codesize
of the implementation library, and should improve the performance
as well, by removing one level of indirection.

This should also be done for test files, but that has lower priority.

This commit also strips the old copyright comment header in touched
files, as those will also be replaced with a more standardized and
machine-friendly version.

This enables us to avoid `catch_reporter_bases.hpp` being indirectly
dependent on `catch_tostring.hpp`, cutting apart quite a bit indirect
inclusions.