The current preferences handling is a mess:
* it's essentially a global config object that anything can modify in any way the caller wants, which is managed across multiple source files which have their own oddities and interdependencies.
* the general preferences has its own bit of SDL event handling and while I get the idea behind `events::sdl_handler` there's no reason to have SDL events handled in the preferences instead of just calling the relevant preferences setter for each event when it happens.
* the general preferences is where most of the preferences are handled and has its `base_manager` struct, which is part of the `manager` struct in the game preferences, which is then implicitly initialized as part of game_launcher's constructor.
* the editor preferences are the only preferences in a sub-namespace `preferences::editor` while all other preferences are just in the `preferences` namespace.
* the display, editor, and lobby preferences are all dependent on including the game preferences, the credentials are dependent on including the general preferences (but not the game preferences), the game preferences are dependent on including the general preferences, and the advanced preferences are entirely their own thing which is dependent on none of the other preference functionality and manages its own singleton.
* nothing checks whether the preferences file has actually been loaded before allowing values to be read from or written to the preferences config - if you attempt to get a value too early in wesnoth's initialization it will silently just give you whatever the default value for that preference happens to be.
With this there is instead a single access point (with exceptions handled via friend functions/classes), all predefined preferences are accessed via their own setter/getter, and all mainline preferences are defined in a single file (preference_list.hpp) so it's easily findable what preferences exist and where they're used. Having the list of all mainline preferences listed out also allows the lua preferences API to provide that full list rather than just the list of the preferences that have been set so far. Also it now checks for whether the location of the preferences file is known before attempting to load the preferences file and asserts if someone attempts to use the preferences too early.
This adds support for having up to 28 distinct sub-achievements within a single achievement. This limit exists since wesnoth's layout isn't smart enough to tell a horizontal listbox to actually use its scrollbar instead of forcing a horizontal scrollbar on the whole window.
Additionally this adds the [set_sub_achievement] and [has_sub_achievement] WML tags and their respective lua functions. [has_sub_achievement] is unsafe for use in MP, for the same reasons that [has_achievement] is.
Instead of being either complete or incomplete, achievements can now specify a value at which they will be considered complete. For such achievements that are not yet complete, a progress bar is added to the achievements dialog showing how close to completion the achievement is.
* 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!
