refactoring: removed Unit._d and renamed dmax to target_d
refactoring: renamed walk_to_xy() to go_to_xy(); now air units use go_to_xy() instead of action_fly_to_remote_target()

Air units still don't have a graph for the allowed exits of a square, so they move in a straight line from the start to the target. While this makes sense, it has the drawback to scatter units between several squares. If the units followed a list of squares, they would always be focused on one square at a time. So maybe later the air units will also have a graph of the possible paths (to model impassable mountains) and the air units will follow the shortest path.
added a menu in the options menu to modify the mods load order

Note: it isn't necessary to save the changes from the options menu anymore.
mods load order option menu: added a submenu to add a mod to the list

This submenu doesn't contain the already selected mods.
mods are automatically loaded after the mods options menu, and when replaying a game with different mods

- now replay briefly announces the map and the players
- now compatibility_version is allowed to change during a client session (removed frozen COMPATIBILITY_VERSION)
- improved mods options menu feedback
added automatically downloadable packages (managed in the options menu)

A package is a zip file which can contain at the same time mods ("mods"), single player maps/campaigns ("single"), and multiplayer maps ("multi").
replaced the mod "crazymod8.2" with an example package: "crazymod"

The package includes not only the crazymod8.2 mod but also the corresponding multiplayer maps.
The URL of the metaserver is http://jlpo.free.fr/soundrts/metaserver/packages.txt .
The URL of the package is, for the moment: http://jlpo.free.fr/soundrts/packages/crazymod.zip .
Now the application makes sure that the files extracted from a package are contained in a folder with the name of the package.