Add CMake doc target

* Added doc/CMakeLists.txt with doxygen target
* Moved Doxyfile to doc/
* Updated Doxyfile (removed obsolete variables)
Merge pull request #302 from diizy/waveshaper

Waveshaper
Merge pull request #297 from Lukas-W/stable-0.4-doc

Add CMake doc target
AutomatableModel: remove bogus assigment if step size is 0

No idea why we set value to m_minValue in case the step size is 0. There
shouldn't be models with step size 0 nevertheless.
AutomatableModel: use nearbyintf() for rounding

For some reason roundf() causes an exception on Win32 when calling with
negative values. This caused LMMS to crash when turning an external MIDI
pitch wheel downwards.

Closes #281.
volumeToMidi(): limit to MidiMaxVelocity
InstrumentTrack, NotePlayHandle: ignore volume of InstrumentTrack for MIDI events

The volume of an InstrumentTrack is applied separately when post-processing
the audio buffer and is not related to MIDI processing. It therefore should
not be included into MIDI velocity calculation.

Closes #301.
SideBar: add tooltips to buttons

Closes #259.
AFP: revert changes
Merge pull request #310 from wongcc966422/issue309

AFP: revert changes
Waveshaper updates: added amplitude +/- buttons, toggle led to clip input to 0dB, more helpful gridlines in wavegraph
Mallets: artwork update by Bill Y.
New knob: knob_vintage32, currently only used in Mallets, can be used for other things in the future (maybe if we get some more vintage-style synths/effects?)
Graph widget: added new drawing mode (bar style graph) for future use, currently unused anywhere, have plans
Slight fix to the wavegraph: grow height by 1 px to make default state look better
Merge pull request #313 from diizy/stable-0.4

Waveshaper updates: added amplitude +/- buttons, toggle led to clip inpu...
README: Add libfltk as build dependency

Needed since 0f4182e084cf5b469b9ff5fb2d693a38871b3947
[skip ci]
Dynamics processor, first commit