Improve PlayHandle removal in Mixer

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## Problem
In ```Mixer::renderNextBuffer()```, when a clear signal is received (by calling ```Mixer::clear()```, which sets ```m_clearSignal``` to ```true```), ```Mixer::clearInternal()``` gets called and it simply adds every ```PlayHandle``` except ```InstrumentPlayHandle```s from ```m_playHandles``` to ```m_playHandlesToRemove```. However, right after ```Mixer::clearInternal()``` is called in ```Mixer::renderNextBuffer()```, a loop iterates through ```m_playHandlesToRemove``` and removes the same ```PlayHandles``` from ```m_playHandles```. In other words, it unnecessarily stores a list of ```PlayHandles``` to be removed only to actually remove then right afterwards. ```Mixer::clear()``` seems to be called only within ```Song::stop()```.

Other than ```Mixer::clear()```, the only method that adds a play handle to m_playHandlesToRemove is ```Mixer::removePlayHandle()```, which is only called by the ```FileBrowser``` class to stop a preset preview ```PlayHandle```. Also, only one preset preview ```PlayHandle``` seems to be active at a time.

## Solution
1. Add the members ```PresetPreviewPlayHandle m_presetPreviewPlayHandle``` and ```bool
m_presetPreviewStopped``` to ```Mixer```.

2. Create the method ```Mixer::addPresetPreviewPlayHandle()```:
```C++
bool Mixer::addPresetPreviewPlayHandle( PresetPreviewPlayHandle *handle )
{
m_presetPreviewStopped = false;
m_presetPreviewPlayHandle = handle;
return addPlayHandle( handle );
}
```

3. Create the method ```Mixer::stopPresetPreview()```:
```C++
void Mixer::stopPreview()
{
m_presetPreviewStopped = true;
}
```

4. Modify ```Mixer::renderNextBuffer()``` so that:
```C++
if( m_clearSignal )
{
m_clearSignal = false;
clearInternal();
}

ConstPlayHandleList::Iterator it_rem = m_playHandlesToRemove.begin();
while( it_rem != m_playHandlesToRemove.end() )
{
// [...]
}
```
becomes something like:
```C++
if( m_presetPreviewStopped && m_presetPreviewPlayHandle != NULL ) {
//Actually stop m_presetPreviewPlayHandle [...]
m_presetPreviewPlayHandle = NULL;
}

if( m_clearSignal )
{
ConstPlayHandleList::Iterator it = m_playHandles.begin();
while( it != m_playHandles.end() )
{
// [...]
}

m_clearSignal = false;
}
```
5. Update ```FileBrowser.cpp``` with the new methods.

Start in Mixer::renderNextBuffer() and Mixer::clearInternal(), then trace the play-handle calls from FileBrowser.cpp and Song::stop(). Compare how clear signals and preset-preview removal currently flow through m_playHandlesToRemove. Done means preset previews stop through the new Mixer methods and clearing no longer queues handles only to remove them immediately.

Written by the indexing model from the issue text.