Add 6 new native building in editor

[dvincent56]

Native building variants in map editor
Adds 6 new native building types to the editor's Native Buildings submenu, all using existing assets:

• Native well (1×1, 3 climate variants)
• Native meeting hut alt (Native_Meeting_Hut_*)
• Native hut alt 2 (Hellenised_Hut_*, 3 design variants per climate)
• Native meeting hut alt 2 (Hellenised_Meeting_Hut_*)
• Native palisade (drag-and-place, reuses Palisade_* with auto-connectivity)
• Native fields submenu with 6 crop variants (wheat / vegetables / fruit / olive / vines / pig) using GROUP_BUILDING_FARM_CROPS

[eugenikus8]

I think the new buildings also need to be added to the src/figure/formation_enemy.c file, in these functions: static int get_structures_on_native_land(int *dst_x, int *dst_y) and static void set_native_target_building(formation *m).

[crudelios]

Can you separate the new buildings from the image loading and create two PRs, please?

This is a giant PR with two entirely different things, separating both would make them easier to review.