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Resolve "Create the plugin_event"

* For `list` and `metric`, filtering conditions can be written using the web2py syntax for database query. Therefore, it can deal with foreign tables
    of the `history` table as well as with the `history.data field`. 
* The `smart_query` is still available but deprecated since it can not handle foreign fields.
* Factorize the application around the `plugin_event`.
* Closes #16 and #19

See merge request !24

Create the modules plugin_tev and the classes EvtModelCore, EvtModelReport, EvtUiCore and EvtUiReport.

Resolve "Improve widgets to filter on data in selector"

* Require `plugin_dbui 0.9.7` or higher
* The selector for report uses the accordion layout.
* The form for event uses the card layout.
* Use the `Table` widget to display event data in he grid showing the event table.
* Closes #18

See merge request !23