ui: Handle snapshots with empty names in snapshot menu

[antangelo]

Snapshots without names are somewhat difficult to create (must use savevm "" in the monitor, QMP, or qemu-img) but should be handled correctly regardless.

This requires adding an additional present bool to the config. I preserved compatibility with existing shortcut bindings by checking the present field or that the shortcut snapshot name string is not empty.

Fixes #1543

[mborgerson]

Should we just prevent creating snapshots with the empty string as the name? This seems like a lot of work to handle this weird case

[antangelo]

We can prevent them from being created within xemu, but that won't cover existing empty-name snapshots or those created by qemu-img (which are also niche cases).