Enhancement to custom jmx provider UI

Currently, each JMX provider has a UI mapped to it, and the UI assumes only 1 icon for each provider.

Fuse Tools has several server types that behave identically, but require different icons. This could be worked around by declaring 4 or 5 different providers, and accompanying xml indicating different icons, but this is a lot of overhead and would make the code much more complex.

A simpler solution is to allow an image provider to be part of the extension point. The image provider could then pull the Image from the server type ui (declared in wtp's api). This would also instantiate fewer images by re-using those already instantiated for wtp.server.