Finish off setting sequence offset user interface

Finish off setting sequence offset user interface
ticket:111
I thought it would be better for the edit sequence properties action to live
under the main edit menu, instead of the popup menu.

Also I decided to use apply/close instead of ok/cancel as it seemed better
to make fiddling with the offset interactive.

And yes I probably should implement some graphical way of aligning the
sequences instead of just typing in offsets.