[CLEANUP] - Refactored code to improve maintainability and facilitate unit testing
- Added parsing methods from String in MarketplaceXmlSerializer
- Moved DocumentBuilderFactory dependency from PluginService and RemoteMetadataPluginProvider to MarketplaceXmlSerializer
- Set IPluginResourceLoader as PluginService instance dependency instead of global static variable access
- Encapsulated getting Pentaho session and PluginManager in an instance method of PluginService to facilitate mocking
- Removed PluginService dependency on PluginFactory, PluginVersionFactory and CategoryFactory.
- Added PluginService dependency on MarketplaceXmlSerializer and IRemotePluginProvider. These are set via decency injection configured in the plugin.spring.xml.template.