Search box/filter added to the instrument list

I've added a search box / filter to the instrument dialog and the new wizard. It is a simple linear search which gives good performance on my computer. This could be improved in the future if needed.

This feature is requested here http://musescore.org/en/node/18542

The filterInstruments() functions is placed in the instrdialog.cpp file, but probably it should be placed somewhere else.

I'm very open to suggestions, improvements, comments, etc.