[Freeciv-Dev] (PR#14650) Patch proposal: unit experience

["Jerome Plut" <Jerome.Plut@xxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=14650 >

Attached: a patch to have the units gain experience, in parallel to the
veteran system.

Diff is against SVN trunk as of last Tuesday.

Changes to gameplay:
 - the units now gain experience points, and can reach experience
   levels (which are totally equivalent to veteran levels),
 - they can also spontaneously gain a level, the raise_chance
   mechanism is still in place (this will set their experience points
   to the number required to achieve next level),
   (this means that you can disable either mechanism via ruleset, to
   keep the one you prefer)
 - it breaks savegame compatibility (for experience is saved in place
   of level),
 - when you mid-click on a unit, it shows the current XP as well as
   the number needed to reach next level.

Change to rulesets:
 - added a "required_xp" integer list to the veteran_system section.
   Any level whose required_xp is -1 is disabled (not reachable
   through XP gain).

What is still to do:
 - decide the appropriate number of XP to gain for each action. I set
   it to ten XP for each (combat, work, high seas), and the levels are
   at 0, 20, 60, 120. This makes the units gain levels at a mean speed
   comparable to that of the veteran_system.