feat: Rename loading dialog per context

[ShrimpCryptid]

Resolves https://github.com/allen-cell-animated/agave/issues/102, "loading dialog: use different titles depending on context."

Solution

• Uses an enum to determine which filetype is being loaded, and changes the title of the loading dialog based on the type.
• Added extra logic when loading recent files to determine the file type, since the information is not stored.

Possible names:

• "Load Settings" (for unknown types)
• "File Load Settings"
• "URL Load Settings"
• "Directory Load Settings"
• "JSON Load Settings" (this usually is not shown because JSON doesn't trigger the dialog).

Screenshots