Create multiparent tasks when drag'n'drop and holding <control>

[Izidor Matušov]

When you do DnD and with <control>, you can usually copy a file instead of moving it. We could adapt this concept in GTG and allow adding a task to a new parent instead of moving.

GTG already supports mutliparent tasks but it is hard to do it if you don't know the trick. (Copy the link to the subtask and put it in another parent)