SetupDialog: Warn of unusual buffersizes

This displays a warning dialog if the users requests unusual
buffersizes:

- buffersizes less than 32
- buffersizes which are not a (natural number) power of 2

This commit also replaces some `setGeometry` stuff by `QBoxLayout`.

include/SetupDialog.h

@@ -102,6 +102,7 @@ private slots:
 	// Audio settings widget.
 	void audioInterfaceChanged(const QString & driver);
 	void toggleHQAudioDev(bool enabled);
+	void updateBufferSizeWarning(int value);
 	void setBufferSize(int value);
 	void resetBufferSize();
 
@@ -179,6 +180,7 @@ private:
 	int m_bufferSize;
 	QSlider * m_bufferSizeSlider;
 	QLabel * m_bufferSizeLbl;
+	QLabel * m_bufferSizeWarnLbl;
 
 	// MIDI settings widgets.
 	QComboBox * m_midiInterfaces;