Fix the ability to sort menu items by priority for CascadingMenu

[cmdcolin]

The @jbrowse/core/ui/Menu has a function for sorting menu items by a priority attribute. The code for doing the same thing for @jbrowse/core/ui/CascadingMenu was missing though. This PR

• adds the same code for sorting by priority to the @jbrowse/core/ui/CascadingMenu (was missing)
• makes it so the "Display types" has a low priority to get sorted last
• re-orders some items in the alignments/pileup display track menu items list