Recording widget/options visibility

Only show the recording widget in the sample clip if the audio engine has
a device configured that can capture audio. For simplicity of the code,
i.e. no nullptr checks, the widget is always created but only shown if
capture capabilities are available.

Rename the context menu entry "Set/clear record" to "Toggle record" and
only enable it if capture is possible.

In `SampleClipView` the test for the availability of a capture device is
abstracted behind the helper method `recordingCapabilitiesAvailable`.

Technical details
------------------
Add the method `captureDeviceAvailable` to `AudioEngine` because clients
should not be concerned with driver details. Drivers should mostly be
used by the audio engine and therefore hidden to clients. Add a private
`const` version of the method `audioDev` so that `captureDeviceAvailable`
can be implemented in a `const` correct way.

Use `captureDeviceAvailable` in the constructor of `SongEditorWindow`.

include/AudioEngine.h

@@ -172,6 +172,8 @@ public:
 		return m_audioDev;
 	}
 
+	bool captureDeviceAvailable() const;
+
 
 	// audio-port-stuff
 	inline void addAudioPort(AudioPort * port)
@@ -355,6 +357,11 @@ private:
 	AudioDevice * tryAudioDevices();
 	MidiClient * tryMidiClients();
 
+	inline const AudioDevice* audioDev() const
+	{
+		return m_audioDev;
+	}
+
 	void renderStageNoteSetup();
 	void renderStageInstruments();
 	void renderStageEffects();

include/SampleClipView.h

@@ -67,6 +67,7 @@ protected:
 private:
 	QWidget* buildRecordWidget(BoolModel& recordModel);
 	void adjustRecordWidget();
+	bool recordingCapabilitiesAvailable() const;
 
 private:
 	SampleClip * m_clip;