Improve attachment saving UX

[jtdaugherty]

Right now attachments get saved to disk in a human-unfriendly location that is difficult to discover, so when users want to save attachments for purposes other than just viewing, that's a problem.

Let's:

• Add a new configuration setting to specify the default attachment saving location
• Add a message selection keybinding to show the list of attached files on the selected message in a popup
• Permit saving attachments from that popup selectively
• When the user chooses to save an attachment, prompt for the destination, defaulting to the location in the configuration, with an option to edit before continuing

[kquick]

Also consider using the new file-selector widget for this (as used for adding attachments to messages).

[kquick]

See also #563.