This commit merges the import and export contexts into a single context
because this is what IOS does, which means we can only reproduce its
behaviour correctly if we use a single context for both operations.
The other reason is that having two separate and very similar structs
is not really a good idea.
While working on this commit, I was notified that our handling of
ImportTmd/ExportTitleInit is not correct. In particular, we always use
the title key for both importing and exporting, which is wrong. To make
this easier to fix in a follow-up PR, the context now also has a title
key field, just like ES. This also lets us avoid computing it every
single time in ImportContentDone.
Léo Lam
dc1707faa8