implement pre-launch ui for configuring iso directory (#301)

UI will automatically open on boot and will not launch the game if there is no valid iso path specified in the config. Can optionally be disabled on future game launches.

src/dusk/settings.cpp

@@ -58,6 +58,11 @@ UserSettings g_userSettings = {
         // Controls
         .enableTurboKeybind {"game.enableTurboKeybind", true},
     },
+
+    .backend = {
+        .isoPath {"backend.isoPath", ""},
+        .skipPreLaunchUI {"backend.skipPreLaunchUI", false}
+    }
 };
 
 UserSettings& getSettings() {
@@ -103,6 +108,9 @@ void registerSettings() {
     Register(g_userSettings.game.midnasLamentNonStop);
     Register(g_userSettings.game.enableTurboKeybind);
     Register(g_userSettings.game.fastSpinner);
+
+    Register(g_userSettings.backend.isoPath);
+    Register(g_userSettings.backend.skipPreLaunchUI);
 }
 
 // Transient settings