Introduced settings database based on sqlite

Potential replacement for QSettings. This database will allow us to save
settings incrementially, removing most of the performance issues we're
having when our few megabytes of settings are being saved.

Not done in the form of a QSettings backend because it doesn't seem the
backends can do an incremental update of the settings.

Currently used by the QuickOpenPlugin for testing.

src/plugins/coreplugin/mainwindow.h

@@ -60,6 +60,7 @@ class ModeManager;
 class ProgressManager;
 class RightPaneWidget;
 class ScriptManager;
+class SettingsDatabase;
 class UniqueIDManager;
 class VariableManager;
 class VCSManager;
@@ -109,6 +110,7 @@ public:
 
     VCSManager *vcsManager() const;
     inline QSettings *settings() const { return m_settings; }
+    inline SettingsDatabase *settingsDatabase() const { return m_settingsDatabase; }
     virtual QPrinter *printer() const;
     IContext * currentContextObject() const;
     QStatusBar *statusBar() const;
@@ -169,6 +171,7 @@ private:
     QList<int> m_globalContext;
     QList<int> m_additionalContexts;
     QSettings *m_settings;
+    SettingsDatabase *m_settingsDatabase;
     mutable QPrinter *m_printer;
     ActionManagerPrivate *m_actionManager;
     EditorManager *m_editorManager;