Introduce SimpleTextFloat

Introduce the new class SimpleTextFloat which simply shows a text and
that works like a pseudo tool tip with a narrow margin. It was extracted
from TextFloat to adhere to the single responsibility principle.

The new class is used by:
* Fader: display the current volume when moving the fader
* Knob: display the current value when the knob is moved
* PianoRoll: display the current value when setting the note volume and
  panning

Add stylings for the new widget in style.css.

include/Fader.h

@@ -55,10 +55,11 @@
 
 #include "AutomatableModelView.h"
 
+
 namespace lmms::gui
 {
 
-class TextFloat;
+class SimpleTextFloat;
 
 
 class LMMS_EXPORT Fader : public QWidget, public FloatModelView
@@ -164,7 +165,7 @@ private:
 	int m_moveStartPoint;
 	float m_startValue;
 
-	static TextFloat * s_textFloat;
+	static SimpleTextFloat * s_textFloat;
 
 	QColor m_peakGreen;
 	QColor m_peakRed;

include/Knob.h

@@ -41,7 +41,7 @@ namespace lmms::gui
 {
 
 
-class TextFloat;
+class SimpleTextFloat;
 
 enum knobTypes
 {
@@ -175,7 +175,7 @@ private:
 	}
 
 
-	static TextFloat * s_textFloat;
+	static SimpleTextFloat * s_textFloat;
 
 	QString m_label;
 	bool m_isHtmlLabel;

include/PianoRoll.h

@@ -60,7 +60,7 @@ namespace gui
 
 class ComboBox;
 class PositionLine;
-class TextFloat;
+class SimpleTextFloat;
 class TimeLineWidget;
 
 
@@ -345,7 +345,7 @@ private:
 
 	static std::array<PianoRollKeyTypes, 12> prKeyOrder;
 
-	static TextFloat * s_textFloat;
+	static SimpleTextFloat * s_textFloat;
 
 	ComboBoxModel m_zoomingModel;
 	ComboBoxModel m_zoomingYModel;

include/SimpleTextFloat.h

@@ -0,0 +1,61 @@
+/*
+ * TextFloat.h - class textFloat, a floating text-label
+ *
+ * Copyright (c) 2023 LMMS team
+ * 
+ * This file is part of LMMS - https://lmms.io
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program (see COPYING); if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301 USA.
+ *
+ */
+
+
+#ifndef SIMPLE_TEXT_FLOAT_H
+#define SIMPLE_TEXT_FLOAT_H
+
+#include <QWidget>
+
+#include "lmms_export.h"
+
+class QLabel;
+
+namespace lmms::gui
+{
+
+class LMMS_EXPORT SimpleTextFloat : public QWidget
+{
+	Q_OBJECT
+public:
+	SimpleTextFloat();
+	~SimpleTextFloat() override = default;
+
+	void setText( const QString & _text );
+
+	void setVisibilityTimeOut( int _msecs );
+
+	void moveGlobal( QWidget * _w, const QPoint & _offset )
+	{
+		move( _w->mapToGlobal( QPoint( 0, 0 ) ) + _offset );
+	}
+
+private:
+	QLabel * m_textLabel;
+};
+
+
+} // namespace lmms::gui
+
+#endif