Usability - start dragging with greater delay.

[tycho-kirchner]

Hi and thanks for this great project, which I've been using for years.
I'm about to switch to the new pygi-python3 branch and noticed a slight regression in comparison to the "old" dockbarx. When clicking on an a group button the mouse has to stand still for a short moment in order to be recognized as a click. However, as a power user, I'm quite fast with the mouse and click on the icon while moving the mouse. This initiates a "drag", while my click is not recognized.
If possible, I suggest to allow dragging (a seldom operation) only after a timeout of maybe 400ms, while everything below should be recognized as a click.

Thanks
Tycho