Fixed difficulty selection popping up multiple times when loading a game (fixes #5392)

The first issue was difficulty_dialog::show being called twice. The second issue was that show_difficulty_dialog
was called from both load_game_ingame and load_game. The latter is called after the former (this should probably
be refactored), so if you selected Change Difficulty when loading from within a game, the dialog prompt would be
shown when both functions were called. Fixed this by toggling off the show_difficulty flag once a difficulty had
been selected.

changelog.md

@@ -22,6 +22,7 @@
    * New `[set_variable]` options: reverse=yes, min=list, max=list
  ### Miscellaneous and Bug Fixes
    * Fixed `[terrain_defaults]` and `[resistance_defaults]` (issue #5308).
+   * Fixed being prompted multiple times to select campaign difficulty when loading a game (issue #5392)
 
 ## Version 1.15.8
  ### Add-ons client

src/savegame.cpp

@@ -101,7 +101,6 @@ bool loadgame::show_difficulty_dialog()
 		}
 
 		gui2::dialogs::campaign_difficulty difficulty_dlg(campaign);
-		difficulty_dlg.show();
 
 		// Return if canceled, since otherwise load_data_.difficulty will be set to 'CANCEL'
 		if(!difficulty_dlg.show()) {
@@ -109,6 +108,7 @@ bool loadgame::show_difficulty_dialog()
 		}
 
 		load_data_.difficulty = difficulty_dlg.selected_difficulty();
+		load_data_.select_difficulty = false;
 
 		// Exit loop
 		break;