Scenario Editor: continue to regroup some static variables in a class,

gfxsheets.hpp: do not remove party sheet when we enter a scenario,
+ minor cleanings...

src/scenedit/scen.townout.cpp

@@ -28,8 +28,7 @@ extern short cen_x, cen_y, overall_mode;
 extern bool editing_town,change_made;
 extern short cur_viewing_mode;
 extern cTown* town;
-extern short mode_count,to_create,cur_town;
-extern ter_num_t template_terrain[64][64];
+extern short cur_town;
 extern cScenario scenario;
 extern cOutdoors* current_terrain;
 extern location cur_out;