Update FxMixer.cpp

prevent double adding of jobs
Check major/minor version before setting theme directory
Merge pull request #1442 from tresf/stable-1.1

Check major/minor version before setting theme directory
Fix scroll bar gap

Closes #1437
Bump DualFilter high cutoff to 20k

Closes #1395.
Merge pull request #7 from LMMS/stable-1.1

merge LMMS/lmms