Improve advancements with icons and percentage of XP increased

[Toranks]

It's a suggestion for an enhancement, not an issue. Example:

[cooljeanius]

AMLAs are defined in units/macros/amla.cfg

[cooljeanius]

possibly-related issue that we might want to watch out for: https://forums.wesnoth.org/viewtopic.php?t=56724

[cooljeanius]

It might also be nice to have wesnoth/wesnoth#7842 fixed for this