Extract BackupPolicy class into its own file to facilitate automated testing

[chennes]

This extracts BackupPolicy to its own file so that it can be tested (it was previously a private class in the Document.cpp file). A test framework is created for the file, and a single test added as a proof-of-concept. Future work is to get this class fully under test before refactoring it. No changes are made to that class in this initial extraction.

As part of this process I reworked some of the C++ test framework to improve the names of the test executables, and to get at least the most basic tests to run on Windows without needing to manually copy any DLL files. Those changes are in a separate commit. There is no longer a "Tests_run" executable because that seemed to imply that it ran all tests, which it did not. It's been split up into four different tests based on the underlying directory structure, and is now "App_tests_run", "Base_tests_run", "Gui_tests_run", and "Misc_tests_run".

Commit ee571ef6e7dfebdd71867cb2d61a891f6ac965eb is the one that actually contains new code that's not just shuffling things around.