enh(core): define a sane default for log file path

Description

Default log file path, when nothing is specified with --log-output in the user "cache" directory, which is given by $XDG_CACHE_HOME on Linux or simply $APPDATA on Windows. As a fallback, the default temporary directory is used. In both cases, the path is suffixed with ./sight/<profile name>. The final path is also displayed on the console.

VTK.log and Ogre.log has been taken into account...

Some (light) adjustment and cleanup have also been made in core::tools::Os to manage "cache" directory and return a std::filesystem::path instead of a std::string.

this could be a possible breaking change as you may have to call std::filesystem::path::string() when calling getUserConfigDir() (already fixed in this MR).

getUserDataDir() name is misleading. Since it returns XDG_CONFIG_HOME it should be called getUserConfigDir(). It was hard to not change this in this MR for me, so it was done....

And last but not least, a small "fix" in PreferencesTest to clean correctly the user config directory after the test pass. I come through it while changing the getUserDataDir() returned type to std::filesystem::path.

Closes #865 (closed)

How to test it?

• Launch an application, as a normal user, without --log-output. The log file should be created in ~/.cache/sight/<profile_name>/sight.<counter>.log on Linux and something like C:\Users\<user>\AppData\Roaming\sight\<profile_name>\sight.<counter>.log on Windows.
• Same as root (Linux) or Administrator (Windows). On Linux, the home change and the path should start with /root/... and should be unchanged on Windows.
• Try the same, but with --log-output. Ensure that --log-output <path> works as expected.