TagPileup GUI tag extend - 2bp by default - should be 1bp

[owlang]

TagPileup default marks 1bp for each encoding and tag extend is how many additional marks should be appended in the direction of the read. Therefore, the default value should be 0 (1 mark + 0 additional marks) but right now it is 1. Functionally, this results in a tally of 2bp long marks (1 mark + 1 additional mark).

Add GUI dialog pop-up for negative tag extend values redirecting the user to use the shift bp feature.

Note: The TagExtend CLI option has already been added with a default value of 0 on dev so just the GUI needs to be adjusted to default to 1.