Fixes Ghost Peak Controller Bug

Fixed. Can now be merged.
----
This pull request fixes https://github.com/LMMS/lmms/issues/115
I am sure that the code is working(at least my local copy is working). I suspect that I screwed the branch because I committed the changes improperly.
I've recreated the branch. Hopefully this time it would work.

EDIT: It's still not working.
~~It's seems that there is a linking error on /lib/libfltk.a . Why does it even happen? I haven't even touched any fltk file.~~
UPDATE(EDIT4): I've misread the error message. Here is the actual error message:
```
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp:108:2: error: a class-key must be used when declaring a friend
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp:108:2: error: friend declaration does not name a class or function
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp: In static member function ‘static bool presetPreviewPlayHandle::isPreviewing()’:
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp:106:9: error: ‘QMutex previewTrackContainer::m_dataMutex’ is private
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp:261:26: error: within this context
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp:106:9: error: ‘QMutex previewTrackContainer::m_dataMutex’ is private
/home/travis/build/LMMS/lmms/src/core/preset_preview_play_handle.cpp:264:16: error: within this context
make[2]: *** [CMakeFiles/lmms.dir/src/core/preset_preview_play_handle.o] Error 1
```
EDIT2: I am trying to build by running the commands shown on Travis CI right now.
EDIT3: The fixes build **successfully** here! I've run the following command to build:
```
git clone --depth=50 git://github.com/LMMS/lmms.git LMMS/lmms
cd LMMS/lmms/
git fetch origin +refs/pull/145/merge:
git checkout -qf FETCH_HEAD
cmake . -DCMAKE_INSTALL_PREFIX=./lmms-bin/
make install
```
Either it's a bug of Travis CI or I get it built with some luck. I am not sure whether it is safe to merge this branch. It's always a good idea to test it before merging it.

EDIT5: Maybe the issue is caused by building in strict mode. After adding the `class` keyword before `friend` , the issue should be fixed.
EDIT6: **Fixed**. Now the branch can be merged.