Restore geometry and state after central widget creation

Attempts to fix a bug where the central page editing widget was sometimes
squashed to a tiny size in the corner.

Also introduces a schema for settings storage. There are no changes in this
schema; this is (mis)used to forget the geometry and state when version
0.8.1 is started for the first time to make sure the central widget is
usuable again.

src/qteletextmaker/mainwindow.h

@@ -113,7 +113,6 @@ private:
 	void init();
 	void createActions();
 	void createStatusBar();
-	void readSettings();
 	void writeSettings();
 	bool maybeSave();
 	void openFile(const QString &fileName);