Popups for File IO stuff

[Kautenja]

• show a popup message when attempting to load a file that is not recognized as an iNES file
• show a popup when failing to load a ROM from a saved state, i.e., a JSON file

the screen of the NES may be used in place of the popup, if necessary.

[Kautenja]

#3 partially resolves the issue by introducing a filter at the filesystem level to prevent loading files that are not ".nes" or ".NES" files. An alert mechanism is still necessary for the above two items; however, as the contents of the files are not filtered for, and the files can still move between Rack sessions.