Plugin API refactoring

[clausnagel]

This PR refactors some parts of the Plugin API:

• The shutdown method has been removed from the Plugin class and moved to the GuiExtension interface (and renamed to shutdownGui). This is to solve shut down plugins in CLI mode? #198
• Two callback methods have been added to the FeatureExportExtension interface: beforeExport and afterExport. They are invoked before an export from the database starts and after the entire export operation has completed. So, they are not invoked for every indivdual feature but only for the entire export operation. A plugin can use these callbacks to make some preparations before an export and to clean up after the export. The afterExport method also reports whether the entire export operation completed successfully or was aborted. This is to solve Extend FeatureExportExtension interface #197
• Similar beforeImport and afterImport methods have been added to the FeatureImportExtension interface.
• The main method process in both FeatureExportExtension and FeatureImportExtension had the same signature. Thus, when implementing both interfaces with the same plugin, there was only one method for both operations. This has been resolved by renaming the methods for each interface.

[yaozhihang]

@clausnagel Thanks for extending and refactoying the Plugin API. All good except a small typo (see my comment)