[FEATURE] Shiftable activities

[mackness33]

Hello!

There are multiple cases where I have to shift a particular activity for a specific amount of time. As of now thought if I shift the start time of a particular activity it wouldn't shift the activities right afterwards in a cascade way.

So I would like to propose the implementation of a flag "anchor" that would divide the activities in two different groups, "flexible" and "fixed".

The result of this feature would be the possibility to shift all the activities until a fixed activity is found. At that point the last flexible activity would have its amount of time from start to end to decrease.
Another cool example would be to increase the current activity of x minutes, this change instead of decrease the time of the next activity it would just shift them of x minutes.

Thanks!