Changes to deprecation logging and deprecate --userdata-dir=relative-path

[jostephd]

See individual commit messages for details.

I'd like to backport the wesnoth.cpp and deprecated.cpp parts. I'm not sure if I should backport the filesystem.cpp changes too.

The filesystem.cpp part is for issue #2000.

[jostephd]

Just noticed that with this change, when I run ./wesnoth -cHeir_To_The_Throne '--campaign-difficulty=3' '--campaign-scenario=19c_Cliffs_of_Thoria' --debug I get warnings about various deprecations, such as:

Invalid WML found: wesnoth.delay has been deprecated indefinitely.
Invalid WML found: wesnoth.deselect_hex has been deprecated indefinitely.
Invalid WML found: wesnoth.is_skipping_messages has been deprecated indefinitely.
Invalid WML found: wesnoth.show_message_dialog has been deprecated indefinitely.
Invalid WML found: wesnoth.skip_messages has been deprecated indefinitely.

These are all from core.lua. The warnings go away if I either remove --debug, set the show_deprecation hidden preference to false, or revert this change (any one of the three).

That hidden preference is off by default so I guess this is actually a good thing, anyone who sets that preference has opted in to chattiness.

[stevecotton]

The commit where this merged to 1.14 seems to coincide with AppVeyor builds failing like this:
WML_tests.cmd : error WML1: Test 'cve_2018_1999023' returned 1, expected 0 [C:\projects\wesnoth\projectfiles\VC12\wesnoth.vcxproj]

A separate error, Error sending Webhook notification: Error calling webhook http://ai0867.net:6661/wesnoth: An error occurred while sending the request. doesn't make the build count as failed, it was already happening on the preceding AppVeyor build.

[jostephd]

The commit where this merged to 1.14 seems to coincide with AppVeyor builds failing like this:

I can't see how this change could cause that, and if the error happens on windows only I can't debug it either. Maybe the tests on windows run wesnoth --debug and there's a level 1 deprecation warning on stderr that makes the test fail?

Anyway, could you open a new issue please, milestone 1.14.10 and blocker? Thanks!