KSP 0.90.0 introduced an improved VAB/SPH editor with part filters for function, module, resources, manufacturers and technology levels.

As a matter of convenience, it would be good to have an SDHI icon to allow users to quickly find and filter for parts used in the Service Module System.

Icons implemented and tested OK, ready for V2.4.

NOTE: The agency file required for this feature causes random part testing contracts to be generated for SDHI, which is undesirable as the SDHI Service Module System stack should be tested as a complete assembly rather than in a piecemeal manner. But as this isn't necessarily a game-breaking issue per se, I'll overlook this for now.