[wpigui] Add platform-specific preferences save

wpigui/src/main/native/include/wpigui.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <functional>
+#include <string>
 
 #include <imgui.h>
 
@@ -123,6 +124,15 @@ void SetStyle(Style style);
  */
 void SetClearColor(ImVec4 color);
 
+/**
+ * Configures a save file (.ini) in a platform specific location. On Windows,
+ * the .ini is saved in %APPDATA%; on macOS the .ini is saved in
+ * ~/Library/Preferences; on Linux the .ini is stored in $XDG_CONFIG_HOME or
+ * ~/.config if the former is not defined. This must be called before
+ * gui::Initialize().
+ */
+void ConfigurePlatformSaveFile(const std::string& name);
+
 /**
  * Emits a View menu (e.g. for a main menu bar) that allows setting of
  * style and zoom.  Internally starts with ImGui::BeginMenu("View").

wpigui/src/main/native/include/wpigui_internal.h

@@ -56,6 +56,8 @@ struct Context : public SavedSettings {
 
   int fontScale = 2;  // updated by main loop
   std::vector<Font> fonts;
+
+  std::string iniPath = "imgui.ini";
 };
 
 extern Context* gContext;