other format for downloadable table

#### Is your feature request related to a problem? Please describe

Our users have problems with the CSV format when they export or download the shown table data. I haven't found an easy solution to this problem within the H2O Wave documentation.

#### Describe the solution you'd like

Is it possible to have csv as the default export format and maybe xlsx as another option?

#### Describe alternatives you've considered

We do have an inconvenient solution as the users need to change the format in Excel and do other steps by themselves.
Or maybe we need to reconsider writing our own export button with the requested format, but we haven't done that yet.

Best regards, Agnes

No source file or test is named. Start by reviewing the H2O Wave documentation and the table download/export entry point to determine how CSV is currently produced; done should mean CSV is the default and XLSX is available as an additional user-selectable format.

Written by the indexing model from the issue text.