filters in variable view

(One of many requests, all together at https://github.com/jonahkichwacoders/PyDev/tree/fabioz_all_requests if it is useful)

Add ability to filter out various types of variables for the variables view

This is particularly useful when running an interactive console session
with IPython and the debugger attached as it allows most of the internal
IPython variables to be filtered away, making it easier to see variables
of interest.