Restrict drill down buttons to only rows or columns

[OMackenzie]

Summary: add an additional setting to the header icons for a visual to only show drill options for rows OR columns instead of having the drill on rows/columns dropdown visible to users.

It is often handy to expose the drill up/down buttons to users. However, if there is a hierarchy in both the rows and columns then the dropdown will appear for the user to select to drill on: rows / columns. This is confusing to less technical users.

In some cases, the rows should always be expanded and it is just the columns that the users should have the option to drill up or down on, but this is not possible with the existing options. 

The dropdown always defaults to rows as well which isn't always the preference.

[swisdom]

At the very least, make it so we can choose the default 'drill on' field (columns vs rows- right now it always defaults to Rows) But yes, making it so we can turn *off* one or the other entirely would be even better!