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.
* Add basic achievements functionality.
This reads the mainline achievements.cfg and then all the achievements of each installed add-on.
This is intentionally handled separately from other WML loading so that:
a) All achievements and their status are able to be displayed on the main menu right after Wesnoth starts and regardless of which add-ons are active.
b) Add-ons can add additional achievements to other content, whether UMC or mainline. For example, a modification that adds more achievements for mainline campaigns.
Marking something as achieved is handled by the new [set_achieved] tag and whether an achievement has been completed can be checked via [has_achievement].
There is no attempt to prevent people from manually editing which achievements they've accomplished.
NOTE: These are *not* in any way related to Steam achievements!
Remove filter.hpp, which wasn't included by any file, and relied on
pane.hpp indirectly via one of the lines removed in this commit. The sorting
functionality seems to have moved into pane.cpp when matrix.hpp was added.
Note: there are two other unrelated `filter.hpp` files.
Header pane.hpp is only used by matrix.hpp, pane.cpp, and window_builder.cpp;
the class is also used via a forward-declaration in window.hpp, which friends
it, allowing a call to window::layout_linked_widgets. The class is also made
accessible via REGISTER_WIDGET.
Part of #6540.
