Add support for bitwise ROM dumps to the Windows MME driver

[sergm]

Since (#44, #56), mt32emu-qt supports bitwise ROM dumps (a.k.a. partial ROM images), those can now be identified, loaded and used by synths.

Should such partial ROM images be configured in the default synth profile, the Windows MME driver shall be able to loads those as well.

Additionally, the driver should not fallback to loading ROM files from the system-wide directory C:/WINDOWS/SYSTEM32/ that may well be not-existent at all. Instead, in case the ROM directory is not stored in the UI application settings, the driver shall fallback to using the implied default path roms in the current user profile, similarly to how the UI application works.