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.
