correct slider init value and better FX slider event handling

Trying to fix issue https://github.com/juliandescottes/piskel/issues/98
2 changes done:
- Setting the value of the range input in the setFPS function so that loading existing framesheets from a service actually sets the slider correctly
- Changing the onchange event listener to oninput in order to change the FPS value while dragging on all browsers (namely on FX)