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.

data/themes/classic/style.css

@@ -70,7 +70,7 @@ QToolTip {
 	color: #4afd85;
 }
 
-lmms--gui--TextFloat {
+lmms--gui--TextFloat, lmms--gui--SimpleTextFloat {
 	border-radius: 4px;
 	background: qlineargradient(spread:reflect, x1:0.5, y1:0.5, x2:0.5, y2:0, stop:0 rgba(0, 0, 0, 255), stop:1 rgba(50, 50, 50, 220));
 	opacity: 175;

data/themes/default/style.css

@@ -100,7 +100,7 @@ QToolTip {
 	color: #d1d8e4;
 }
 
-lmms--gui--TextFloat {
+lmms--gui--TextFloat, lmms--gui--SimpleTextFloat {
 	background: #040506;
 	color: #d1d8e4;
 }