Create better UI for desktop start screen assembly adding

[cmdcolin]

Currently the form for adding a new assembly is a little daunting. might be nice if it was streamlined a little, so people can "Just open a FASTA file". currently you are required to specify the assembly name for example, where it could be inferred from filename if not specified

user in office hours expressed confusion

also, having more desktop oriented docs could help, but just a better UI is probably the best scenario