Reduce Config Writes

• The INI configuration file is written in three stages. Firstly, a temporary file is written besides the original file. Secondly, the original file is deleted. Thirdly, the temporary file is renamed to the original file. The file rename operation should be atomic (on most filesystems). Should this sequence of operations be interrupted the original INI file would be left intact or the new complete configuration would be available in a temporary configuration file.
• Changes to the INI configuration are written immediately after respective changes are made. This uses the signal-slot mechanism.
• Configuration saving is now clearly divided into INI-based configuration and preferences which are stored in a SQLite database.
• Only window geometry is explicitly stored in main window destructor.
• When the application is started if the main dsgui.conf is missing the application searches for an auxiliary configuration file. If such is found it is renamed to dsgui.conf.

addresses #699 (closed)

closes #699 (closed)