refactoring, icons

In the last days, I did some major code cleanup, to make extensions
easier. The class hierarchy is a little enriched to avoid switch cases
in collections, and factory classes provide information for all objects
and actions so adding new code has minimal impact (no more editing
frmMain/event/menu/misc/pgObject/whoKnowsAllThePlaces)
As a result, I might have broken some checks, so please report all
oddities/problems/bugs you notice.

In particular, the following problems may occur:
- menu items not enabled/not disabled when appropriate
- wrong results after refresh
- wrong icons

Thanks to Niko, we have shiny new icons for all pgAdmin objects,
including all collections. IMNSHO he did a very decent job, thanks Niko!

Regards,
Andreas