Wrangle sample method selection dialog

Modify the sample dialog to provide the following options:
1. First N rows
2. Ratio random rows of a % of the dataset
3. Ration (#) random rows of of the dataset

The latter two provide a way to return a statistically representative sample of the dataset at the expense of speed of the sampling operation itself. Indicate to user the choice will cause engine to perform 1-2 passes on the dataset. Big data the operation could be long.