Merge the two scenario selection dialogs

Now both preset and custom scenarios are joined using the "Start Scenario" button.
Preset scenarios still get special treatment.

src/game/boe.fileio.hpp

@@ -23,8 +23,8 @@ void import_anim_terrain(short mode);
 void start_data_dump();
 void end_data_dump();
 short onm(char x_sector,char y_sector);
-void build_scen_headers();
-bool load_scenario_header(fs::path filename/*,short header_entry*/);
+std::vector<scen_header_type> build_scen_headers();
+bool load_scenario_header(fs::path filename,scen_header_type& header_entry);
 fs::path locate_scenario(std::string scen_name);
 
 void alter_rect(rectangle *r);