Make it possible to target expanded scheduler with CSS selectors

[denis-taran]

Currently it is not possible to apply a different style to the scheduler based on the state of the expand plugin via CSS selectors, because all changes are made by modifying style properties directly. This change will add dhx_scheduler_expanded class every time the scheduler is expanded and remove it, when the scheduler is collapsed. I didn't use the classList property, so it should work even with older browsers. After the change, scheduler styling can be modified via CSS selectors like this:

.dhx_scheduler_expanded .dhx_cal_date {
  font-size: 20px;
}