make chooser display extent of dna to be exported on the zmap, make dialog non-modal, make spinners consistent.

add code to draw an ovelay box and to change a boxes size + make box parent a group in params. list.

removed some old code and renamed functions

add code to support marking of features and ranges. Add function to convert between world coords and sequence.

add some functions to get sequence bounds and world bounds from items. Add funcs to support marking items.