Is it possible to let the user resizing the cells manually (using the mouse)?
Unfortunately, that is not supported.
OK, are there any plans to support it in the future?
It's not trivial and it has various implications - e.g. it can be either non-persistent or it would require a manual timeline. Also, it's necessary to take into account that the time header columns don't have to correspond to the grid cells and so on.
So it's not in the plan at the moment. If you'd like to discuss the possibility of sponsoring this feature, please contact firstname.lastname@example.org.