This is a followup to #1485

See existing functionnality in API.get* methods from the API page.

Metadata improvement list:

• include per goal conversions, revenue and conv. rate for each segment
• add Column description (on hover) in the metadata
• add footer message
• add Related Reports list (eg. see Actions/Controller or Marketshare reports)
• refactor the SEO API to make the datatable compatible and gettable (should return a simple array of column->values )
• refactor controllers and templates to exclusively use metadata
• Should functions like getNumberOfDistinctCampaigns be separately as a metadata, or grouped in a function returning all values at once?
• Add the notion of Units in the metadata? (time, percentage, revenue, plain)
• (Major) Add support for date ranges / time periods (for Piwik Mobile Line Graphs and other evolutional views)

The final state could be that each plugin defines in their metadata the default display options for each report. We would then have to set this array of options to the ViewDataTable which would call the specific functions setColumnSorted() setColumnsToDisplay() setFooterMessage() etc.

Other UIs (like Mobile Piwik client) and other report exports (like PDF reports) could benefit from these as well, which would also ensure consistency and make Piwik reports clear to understand.