New table type for table data lazy loading

[hanjinliu]

A lazy loader table widget is indispensable for big data visualization. It is also very useful if a very large data is saved in separate csv files.

Passing dask.dataframe is not the solution because iloc is poorly supported.

• It should support: find, sparse edit, coloring, formatting, validation
• It should support under restricted context: plot, summarize
• It should NOT support: sort, copy all, reshaping, concatenation