Fix bug: fps rounding to integer - ID: 3104763

Fix bug: fps rounding to integer - ID: 3104763
Also fixed that widget time slider frames per second was not updated when changing the render description.
It produced bad time cursor position when dragging the time cursor and the document fps is not 24 fps (the default one for widget time slider)
Now when the render description is modified, the sock time track widget time slider fps is updated.