Add missing Q_OBJECT macros in header files

Fixes 20 warnings similar to the following ones when generating a translation file with e.g. "make de.ts":
/home/daniel/Lmms/src/tracks/AutomationTrack.cpp:40: Class 'AutomationTrack' lacks Q_OBJECT macro
/home/daniel/Lmms/src/core/FxMixer.cpp:41: Class 'FxRoute' lacks Q_OBJECT macro
/home/daniel/Lmms/src/core/FxMixer.cpp:554: Class 'FxMixer' lacks Q_OBJECT macro
/home/daniel/Lmms/src/gui/widgets/MeterDialog.cpp:52: Class 'MeterDialog' lacks Q_OBJECT macro
...