Change seed value manually at ScanWidget

[Aijuh]

Feature you want to implement

In the Scan widget, user can choose whether or not to randomize the sequence.
In this step, seed argument is supported at artiq master as below.
"scanEntry": {"npoints": 3, "randomize": 2, "seed": null, "start": 2.0, "stop": 12, "ty": "RangeScan"}

Since giving user an option to select seed value is more preferable, we will add option for chaging seed value manually after discussion.

[BECATRUE]

This is also an important feature, but the original functions i.e. supporting other scan types is more preferred, thus please implement the originals first.