TypeSelector and new item dialog are slow because they create instances

[gkronber]

This should be solve by detecting and listing only the available types. However, the image to display must be available so a static image property for items must be introduced. The virtual item image property should return the static image by default. But this behavior can be overridden in derived classes. If the static property is not available for an item the image of the base class should be discovered by a base class traversal in the default implementation for the item image property.

[swagner]

Implemented static item image properties to avoid instance creation in TypeSelector and NewItemDialog in r7201.

[ascheibe]

I have reviewed and tested the changes, looks good, very nice speed-up!