Make "global plugins" for JBrowse Desktop

[garrettjstevens]

As discussed in grooming, global plugins would be loaded when the start screen is loaded on desktop, meaning there will need to be a plugin loader created then. These plugins would then be merged into any newly created sessions from the start screen. This way users would not have to install commonly used plugins separately every time from each session. This would also allow the opportunity for global plugins to modify the start screen, for example Apollo could use this to allow a user to open a GFF3 for annotation from the start screen.