Improvements and fixes to Import & Export

New Features

• Added a the option to include Cover art in export.

Improvements

• Improved performance of zip and unzip. This speeds up import, export and backup.

Fixes

• Fix included files in Satchels not being imported.

Road to cross platform

• Adopt Mvvm Toolkit (to prepare for switch to AvaloniaUI).
• Switch Zip library from windows only DotNetZip to Cross platfrom SharpCompress.
• Move all preferences to the preferences.xml file.