More prominent recording option

Make the option to record samples more prominent by providing a pixmap
button that is always shown and that can be used to turn recording on or
off. The corresponding widget which constitutes of the button and a label
is always positioned at the bottom left of the sample clip.

Technical details
------------------
The recording widget is built in the private method `buildRecordWidget`.
The method `adjustRecordWidget` is used to move the recording widget to
the correct position relative to the sample view. It is called after
construction and whenever the sample clip is resized (see `resizeEvent`).

Add the method `SampleClip::getRecordModel` so that the pixmap button can
be configured to act on it.

include/SampleClip.h

@@ -81,6 +81,8 @@ public:
 	void setIsPlaying(bool isPlaying);
 	void setSampleBuffer(std::shared_ptr<const SampleBuffer> sb);
 
+	BoolModel& getRecordModel() { return m_recordModel; }
+
 public slots:
 	void setSampleFile(const QString& sf);
 	void updateLength();

include/SampleClipView.h

@@ -27,10 +27,13 @@
 
 #include "ClipView.h"
 
+class QWidget;
+
 namespace lmms
 {
 
 class SampleClip;
+class BoolModel;
 
 namespace gui
 {
@@ -59,12 +62,18 @@ protected:
 	void dropEvent( QDropEvent * _de ) override;
 	void mouseDoubleClickEvent( QMouseEvent * ) override;
 	void paintEvent( QPaintEvent * ) override;
+	void resizeEvent(QResizeEvent *event) override;
 
+private:
+	QWidget* buildRecordWidget(BoolModel& recordModel);
+	void adjustRecordWidget();
 
 private:
 	SampleClip * m_clip;
 	QPixmap m_paintPixmap;
 	bool splitClip( const TimePos pos ) override;
+
+	QWidget* m_recordWidget;
 } ;