TextEditorPlugin: Pimpl and remove uses of global object pool

Change-Id: I17eb0e928788f02f6089fd2a2ed2393d7f577d11
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/texteditor/texteditorsettings.cpp

@@ -81,8 +81,7 @@ public:
 static TextEditorSettingsPrivate *d = 0;
 static TextEditorSettings *m_instance = 0;
 
-TextEditorSettings::TextEditorSettings(QObject *parent)
-    : QObject(parent)
+TextEditorSettings::TextEditorSettings()
 {
     QTC_ASSERT(!m_instance, return);
     m_instance = this;