Don't let people connect to `{file}`

[richb-hanover]

OK. I figured out how I got into such a bad state (#47). Here's my thought process that led me to the bad state.

• Server -> Add Server... "Hmmm... It's not really a server. But I think I know what qstudio means."
• Sever Ty... "Hmmm... I don't know what a 'KDB' is, but look! There's SQLite. I'll choose that."
• Connect By: "I dunno...
  • "It's not a host on the network: it's just a SQLite file. So I won't click Host"
  • "I guess it must be a URL. But that seems like it would have to be on the network as well. Well, the default is usually safe.
  • jdbc:sqlite:{file} "I don't know what that means, either. I'll leave it alone"
• Name: "I guess I need to type the name of the file to open"
• "I'll just leave all the rest empty/as-is"
• Blammo! qstudio creates the blank {file} file, and life stinks.

To refresh my memory of how this looks on my Mac, I posted a video. It's at: https://youtu.be/sw9nZXIRzZo

Suggestions:

• SQLite databases are odd: they wrap everything into a single file. If I select "SQLite", maybe qstudio could display a file picker for the "Connect By" section?
• If someone leaves the choice as jdbc:sqlite:{file}, it would be good to give an error, saying to enter a file name not "{file}"