Moved the copying of themes from the build script to the project file.
The recursive copy-item call in the post-build script was causing the
2 files inside `Themes/ThemeBuilder` to also be copied, which resulted
in the `Tempalte.xaml` being loaded at runtime as a theme.
Closes#38
* use dotnet pack for the plugin project
* refactor Zip-Release and Pack-Squirrel-Installer functions
* remove reference to NuGet.CommandLine - no longer needed
- ppy.squirrel is not fully .net core upgraded, so it is still showing restored using .net framework
- does not contain .net core runtime installer which is now required by Wox
- UpdateManager.RestartApp method is throwing an OSU application specific error which Wox is unable to resolve.
- Squirrel.Windows 1.5.2 still works and functionalities required by Wox are compatible. Tested RestartApp method, tested enable/disable protable mode methods and tested updating Wox to the latest version.