v4: option toolbarPlacement

The position of toolbar is floating: at the bottom in calendar mode and at the top in clock mode. This confuses some users. So I added option `toolbarPlacement` with values 'default', 'top' or 'bottom'.
Also the widget added CSS class `usetwentyfour` in side-by-side mode only. I changed the behaviour, so `usetwentyfour` is always added now (if `use24hours` flag is true). And I decreased the height of `td.hour` in 24-hour mode (in LESS). Otherwise the clock selector is too high.