For non-Windows platforms this adds:
* a new check for whether the logs directory exists and is writable by attempting to write a single whitespace to a dummy.log file.
* a popup shown immediately after the GUI system is initialized letting the player know if there was an issue creating the log file.
For Windows, this changes:
* instead of writing to the OS temp directory and then attempting to move that file to the logs directory, this uses the same check that was described above for non-Windows platforms.
* the alert shown when writing the dummy log file fails is now a Wesnoth alert message rather than the Windows-specific MessageBox.
* a failure to create a log file does not immediately exit wesnoth anymore.
Additionally, this makes it so that for the default state (logging to file), all platforms follow the same code path by calling `lg::set_log_to_file()`. `log_windows` now contains only the Windows-specific logic needed for handling the creation/redirection of output to a console.
