Add API for saving settings with default value

We should never actually write default values into the settings, because

- if the default value changes in a later Qt Creator version, the new
  default should automatically take effect if the user didn't change the
  value
- it senselessly grows the settings file

Add a QtcSettings class that extends QSettings by a
"setValueWithDefault" method, which does not write default values to the
settings, and actually removes the settingskey if the user switches back
to the default.

Use it at the places where we already do this manually.

Task-number: QTCREATORBUG-24762
Change-Id: Ia76414cb21e8521f3aeed1e37b43ae4fb3393ea3
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/vcsbase/commonvcssettings.cpp

@@ -62,7 +62,7 @@ CommonVcsSettings::CommonVcsSettings() :
 {
 }
 
-void CommonVcsSettings::toSettings(QSettings *s) const
+void CommonVcsSettings::toSettings(Utils::QtcSettings *s) const
 {
     s->beginGroup(QLatin1String(settingsGroupC));
     s->setValue(QLatin1String(nickNameMailMapKeyC), nickNameMailMap);
@@ -70,11 +70,9 @@ void CommonVcsSettings::toSettings(QSettings *s) const
     s->setValue(QLatin1String(submitMessageCheckScriptKeyC), submitMessageCheckScript);
     s->setValue(QLatin1String(lineWrapKeyC), lineWrap);
     s->setValue(QLatin1String(lineWrapWidthKeyC), lineWrapWidth);
-    // Do not store the default setting to avoid clobbering the environment.
-    if (sshPasswordPrompt != sshPasswordPromptDefault())
-        s->setValue(QLatin1String(sshPasswordPromptKeyC), sshPasswordPrompt);
-    else
-        s->remove(QLatin1String(sshPasswordPromptKeyC));
+    s->setValueWithDefault(QLatin1String(sshPasswordPromptKeyC),
+                           sshPasswordPrompt,
+                           sshPasswordPromptDefault());
     s->endGroup();
 }