[wpigui] Add hooks for custom load/save settings

Add GetPlatformSaveFileDir().

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

@@ -79,6 +79,36 @@ void AddEarlyExecute(std::function<void()> execute);
  */
 void AddLateExecute(std::function<void()> execute);
 
+/**
+ * Customizes save/load behavior.
+ *
+ * By default, the integrated ImGui functions are used for this;
+ * ImGui::LoadIniSettingsFromDisk(io.IniFilename) is called at startup, and
+ * ImGui default automatic save file handling is used via io.IniFilename.
+ *
+ * Calling this function results in the load function being called at startup,
+ * io.IniFilename set to null (which disables ImGui's integrated file saving),
+ * and the save function being called when io.WantSaveIniSettings is true.
+ * The loadIni function should call ImGui::LoadIniSettingsFromMemory() to load
+ * ImGui save data, and the save function should call
+ * ImGui::SaveIniSettingsToMemory() to get ImGui save data.
+ *
+ * The load function is called PRIOR to AddInit() functions, and the loadIni
+ * function is called AFTER to AddInit() functions.  This allows initialize
+ * functions that use custom storage to handle the loaded values, and initialize
+ * functions that use INI storage to add hooks prior to the load INI occurring.
+ *
+ * This must be called prior to Initialize().
+ *
+ * @param load load function
+ * @param loadIni load INI function
+ * @param save save function; false is passed periodically, true is passed once
+ *             when the main loop is exiting
+ */
+void ConfigureCustomSaveSettings(std::function<void()> load,
+                                 std::function<void()> loadIni,
+                                 std::function<void(bool exiting)> save);
+
 /**
  * Gets GLFW window handle.
  */
@@ -137,6 +167,14 @@ void SetStyle(Style style);
  */
 void SetClearColor(ImVec4 color);
 
+/**
+ * Gets the (platform-specific) absolute directory for save files.
+ *
+ * @return Absolute path, including trailing "/". Empty string if directory
+ *         could not be determined.
+ */
+std::string GetPlatformSaveFileDir();
+
 /**
  * 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

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

@@ -40,6 +40,9 @@ struct Context : public SavedSettings {
 
   GLFWwindow* window = nullptr;
 
+  std::function<void()> loadSettings;
+  std::function<void()> loadIniSettings;
+  std::function<void(bool exiting)> saveSettings;
   std::vector<std::function<void()>> initializers;
   std::vector<std::function<void(float scale)>> windowScalers;
   std::vector<std::pair<