Centralize legacy preference storage

src/legacy_preference_storage.h

@@ -0,0 +1,10 @@
+#pragma once
+
+namespace pp::panopainter {
+
+void set_legacy_integer_preference(const char* key, int value);
+void save_legacy_boolean_preference(const char* key, bool value);
+void save_legacy_float_preference(const char* key, float value);
+bool save_legacy_preferences();
+
+} // namespace pp::panopainter