Add Open file option from landing/start screen on JBrowse desktop

[carolinebridge]

Currently, there is no way to open a .jbrowse file directly from the landing/start screen on JBrowse desktop (to be specific, a .jbrowse file that has not been opened before).

Currently, if a user would like to do this (without navigating to the file's location in their directory and opening it manually), they'd have to create a new session > File > Open > select their .jbrowse file.

It's a little inconvenient and not conducive to sharing; it's very likely a user's workflow for opening a shared .jbrowse file they've never opened before would be Open JBrowse desktop > Open shared file. A user less familiar with JBrowse might get stuck here -- not know that they can open a quickstart assembly and then access the File > Open option, or not know they can double click .jbrowse files.

Add an "Open .jbrowse file" or "Open config file" option directly on the landing/start screen, could fit in either panel, maybe better suited to Launch new session.