Edit an archive opened inside an archive

[sarensw]

An archive inside an archive opens in the same window. Since #267 nothing in it can be changed there: its entries belong to the inner archive, and a save writes only the outer one. Deleting or adding inside it used to change the outer archive instead, and hit the wrong entries.

Editing it for real means:

• writing the inner archive, which lives as a temporary copy extracted from the outer one;
• putting the written copy back into the outer archive, replacing its entry;
• one Save that writes both, inner first, and refuses when either cannot be written;
• the inner archive's own password and engine, which the window already keeps per archive;
• the inner archive being in a format MacPacker can write — zip today, 7z with Edit existing 7z archives #265.

It is the same kind of work as editing in place in general: one change that has to go through more than one archive.

Found while working on #267.