Implemented prototype module for viewing overall event statistics.

The Dashboards module in Hawat user interface now contains prototype implementation for calculating and viewing overall event processing statistics. The module design is based on original Perl-based interface with many enhancements both on the surface (interface) and in the backend code. The work is not yet finished, some of the tasks that remain to be done are:

* Links from data tables to other modules of the interface.
* Missing sorting arrows in data table headers.
* Interface tweaks.
* Chart descriptions and/or help texts.
* Search form tweaks or changes (current implementation is maybe too powerfull for general user).

(Redmine issue: #3734)