Create a button to link/de-link Planned start and Due date
Currently whenever I change the due date the planned start date will move as well.
This might be correct sometimes, but isn't in other cases, because sometime you might want to change the start/end date including the duration, while in other instancies you might want to just move the task (=change the other date simultaneously)
I propose to introduce a button in form of two chain elements which are either tied together or apart and by this either change the complementary date as well or not.
This is similar concept as used by some graphics programs (eg. GIMP, Layer->Set Layer Boundary Size) to link height and width dimensions (proportional/no proportional).
The idea of per-task option is fine. Note, however, that the global connection is a preference setting. You can already turn off the connection for everything in the preferences, and it should have been off by default at first.