Create Quickload Saver IGB app

IGB makes it possible for users to visualize "custom genomes," which are genomes where they have used their own reference genome and visualized their own data files.

Please see this User's Guide page for guidance on how this features works from the user's perspective:

https://wiki.transvar.org/pages/viewpage.action?pageId=23200542

Unfortunately, each time a user needs to use their "custom genome," they have to repeat the entire process described in the wiki.

Instead of making them do this every time, let's implement a way for them to save their custom genome between sessions.

How this could work:

• Users will do the same thing as now when opening their custom genome.
• We will create a new submenu option under the "File" menu that says "Save Custom Genome to Local Quickload Site"
• When they choose that option, a popup will appear that asks the user to select a folder in the file system
• Once they do that, IGB creates a quickload site consisting of meta-data files only - contents.txt, genome.txt, annots.xml
• The annots.xml file tag "file" attribute will point to the local (or remote) files the user has loaded into IGB

Note that thanks to the new "reference" file tag implemented in IGB 9.1.6, the reference sequence no longer has to reside in the same folder as the Quickload site, as was previously the case.