[#2550] Add touch and copyFile to Filesystem.

Problem
=======

We would like to have touch and copyFile method to compat filesystem.


Changes
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Implement copyFile.

Touch was already implemented but as private method only used in compat
tests.


How to test
==========

reviewers: @alibotean

check that changes make sense.

True... I have renamed the test to `test_copyFile_no_explicit_name`