This PR fixes issues on systems where `QCursor::setPos()` has no effect
or is not reliable. These issues included knobs moving to fast on some
operating systems. Affected widgets are `Knob` and `LcdSpinBox`.
With this PR, on all operating systems, the `setPos` calls are removed and
the cursor is not hidden anymore, so the mouse keeps moving normally
when changing values of one of the widgets.
As now the previous pointer position keeps moving (instead of being reset
to the original position using `QCursor::setPos`), the mathematics that
translate pointer pixel distance to `Knob`/`LcdSpinBox` value increase
have to be changed:
* The `Knob` transition function is now linear and uses a new factor.
* `LcdSpinBox` now uses float values and saves the current float remainder
(this is actually a separate issue revealed by this fix), leading to a fluent,
non hanging movement.
Moving empty destructors out of the .cpp files and into headers
allows them to be devirtualized in certain cases.
(When the compiler can't "see" a function in a header, it must largely
assume it's some black box that the linker will resolve.)
While we're at it, use C++11's `= default` to define empty virtual
desturctors for us.
For some classes (e.g., Piano), nothing is derived from it, so we can
mark the class as final and remove any explicit virtual dtor.
There are many other places where this can be done, but this is a large
enough patch as-is.
