Pull the class OutputSettings out of the class ProjectRenderer so that
it can be used in other contexts as well. Also move the enum
ProjectRenderer::Depth into the new class OutputSettings and rename it
to BitDepth. Adjust all places that referenced
ProjectRenderer::OutputSettings accordingly.

Adjust the two places where an instance of OutputSettings is created:
the main function and ExportProjectDialog::startExport.

Store an instance of OutputSettings in AudioFileDevice and remove
several members and methods which are now replaced by this instance. Add
a getter for the OutputSettings to AudioFileDevice. Storing an instance
of OutputSettings in the base class AudioFileDevice enables the
simplification of the following constructors and general code in the
following classes:
* AudioFileDevice
* AudioFileOgg
* AudioFileWave

Because OutputSettings contains everything related to sample rate, bit
rate settings and bit depth these parameters could be removed from the
parameter list of the aforementioned constructors.

Simplify the signature of the factory method AudioFileDeviceInstantiaton
(defined in AudioFileDevice.h) and reorder the parameters by significance.

Move the logic of how the minimum and maximum bitrate is calculated
using the nominal bitrate into AudioFileOgg::minBitrate() and
AudioFileOgg::maxBitrate(). Previously this was defined in the
constructor of ProjectRenderer where it does not belong as it an
implementation detail of the OGG export.

Remove the code that converted the bit depth enum to an integer from
ProjectRenderer as it is now solely represented as an enum.

Remove class members for the minimum and maximum bit rate from
AudioFileOgg and adjust the code in the implementation to use the values
stored in OutputSettings.

Only show widgets on the export dialog that are relevant to the selected
file format (Wave/Ogg):
* Sample rate is always shown.
* Bit depth settings are only shown when Wave is selected.
* Bit rate settings are only shown when Ogg is selected.

Remove the label that informs the user that not all settings apply to
all export formats as it is not needed anymore. The english text of that
label was: "Please note that not all of the parameters above apply for
all file formats."

If the variables bit rate is not enabled the nominal bit rate will be
used for the minimum and maximum bit rate in the encoder.

If the variable bit rate is enabled the current implementation will
compute the minimum bitrate by subtracting 64 kBit/s from the nominal
bit rate. The maximum bit rate is computed by adding 64 kBit/s to it.

Example: The nominal bit rate is set to 160 kBit/s and variable bit rate
is enabled in the export dialog. The minimum bit rate is then set to 96
kBit/s and the maximum bit rate to 224 kBit/s.