settings: Only save settings for visited pages

Set a flag when a page is shown. On apply or accept, only save if that flag is
set.

src/ui/settingspage.cpp

@@ -17,18 +17,35 @@
 
 #include "settingsdialog.h"
 #include "settingspage.h"
+#include "core/logging.h"
 
 SettingsPage::SettingsPage(SettingsDialog* dialog)
-    : QWidget(dialog), dialog_(dialog) {}
+    : QWidget(dialog), maybe_changed_(false), dialog_(dialog) {}
+
+void SettingsPage::showEvent(QShowEvent* event) {
+  QWidget::showEvent(event);
+  maybe_changed_ = true;
+}
 
 void SettingsPage::Apply() {
-  Save();
+  if (maybe_changed_) {
+    qLog(Debug) << "Saving" << windowTitle();
+    Save();
+    if (!isVisible())
+      // Don't expect additional changes until the page is visible again.
+      maybe_changed_ = false;
+  }
 }
 
 void SettingsPage::Accept() {
-  Save();
+  if (maybe_changed_) {
+    qLog(Debug) << "Saving" << windowTitle();
+    Save();
+    maybe_changed_ = false;
+  }
 }
 
 void SettingsPage::Reject() {
   Cancel();
+  maybe_changed_ = false;
 }

src/ui/settingspage.h

@@ -45,10 +45,15 @@ class SettingsPage : public QWidget {
   // The dialog that this page belongs to.
   SettingsDialog* dialog() const { return dialog_; }
 
-signals:
+ signals:
   void NotificationPreview(OSD::Behaviour, QString, QString);
   void SetWiimotedevInterfaceActived(bool);
 
+ protected:
+  void showEvent(QShowEvent* event);
+
+  bool maybe_changed_;
+
  private:
   virtual void Save() = 0;
   virtual void Cancel() {}