Show an error when mainline campaigns are missing.

A Linux packaging issue has, for apparently multiple years now, resulted in Wesnoth getting installed with only the tutorial campaign available. So, check for that and alert the player that there's an issue so they don't think Wesnoth actually only has a tutorial available.

Fixes wesnoth#7875

src/gui/dialogs/campaign_selection.cpp

@@ -17,9 +17,11 @@
 
 #include "gui/dialogs/campaign_selection.hpp"
 
+#include "filesystem.hpp"
 #include "font/text_formatting.hpp"
-#include "gui/dialogs/campaign_difficulty.hpp"
 #include "gui/auxiliary/find_widget.hpp"
+#include "gui/dialogs/campaign_difficulty.hpp"
+#include "gui/dialogs/message.hpp"
 #include "gui/widgets/button.hpp"
 #include "gui/widgets/image.hpp"
 #include "gui/widgets/listbox.hpp"
@@ -288,6 +290,12 @@ void campaign_selection::filter_text_changed(const std::string& text)
 
 void campaign_selection::pre_show(window& window)
 {
+	std::vector<std::string> dirs;
+	filesystem::get_files_in_dir(game_config::path + "/data/campaigns", nullptr, &dirs);
+	if(dirs.size() <= 1) {
+		gui2::show_error_message(_("Most mainline campaigns are missing. If you’re on Linux, then this means the incorrect package was installed. To get the rest of the campaigns you will need to either: correctly install the wesnoth metapackage, install each campaign’s package one by one, or install Wesnoth via Steam or Flatpak."));
+	}
+
 	text_box* filter = find_widget<text_box>(&window, "filter_box", false, true);
 	filter->set_text_changed_callback(
 			std::bind(&campaign_selection::filter_text_changed, this, std::placeholders::_2));