fix compiler warning due to -Wenum-compare
most importmidi_*.h don't need to get MOCed

this prevents a message "Note: No relevant classes found. No output
generated." during build, at least under Windows and using QtCreator
avoid compiler warning due to -Wunused-but-set-parameter

which seems to show up only when building under/for Windows
Merge pull request #897 from Jojo-Schmitz/warnings

Fix some warnings