Widgets: Implement counter widget

[offtherailz]

Description

As for the mockup.
The counter widget should provide an aggregated (sum, avg, min, max...) value about an attribute, like this:

Issues

• Implement Geodashboard #2622
  Acceptance criteria:
• the counter must be present on both dashboard and map contexts, with map sync as for the charts.
• counter creation/editin wizard should allow to configure a variable, the aggregation function to use and optionally a filter for the data to use. It allows also to confiugre the unit of measure to show

[chiaracurcio]

Note this observation:
when you create a COUNTER WIDGET on dashboard and you open the menù of the widget you have this 3 options:

if you select the first one "show chart data" you have not any result, i suppose that this button is not correct for this widget
note also that it is different in mokup
note also that this happens also adding a COUNTER WIDGET on the map.