Implement Interface for External Data Source import into Calc

My project in GSoC was to implement a usable interface for Data Providers and adding transformations . What is Data Provider ? Data Provider can be accessed by going to Tabbed UI -> Data -> Data Providers in Libreoffice Calc . It can also be customised into any toolbar and menu . It supports importing external data from various sources. The external data currently supported are CSV , HTML , XML and Base . Data can be imported either from local storage such as CSV files or from the  external sources such as HTML Web Pages . Data can also be manipulated using various transformations before loading it into the sheet . What are Data Transformations ? After importing data , various transformations such as Delete Columns , Sort Columns , etc. can be applied to the data . This is useful in the scenarios when users want to modify the data according to their convenience before importing .  The project is divided into three phases - 1. Implement UI for Data Providers . 2. Adding T...