* 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!
