IOS/ES: Merge the title import and export contexts

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.

Source/Core/Core/State.cpp

@@ -73,7 +73,7 @@ static Common::Event g_compressAndDumpStateSyncEvent;
 static std::thread g_save_thread;
 
 // Don't forget to increase this after doing changes on the savestate system
-static const u32 STATE_VERSION = 87;  // Last changed in PR 5707
+static const u32 STATE_VERSION = 88;  // Last changed in PR 5733
 
 // Maps savestate versions to Dolphin versions.
 // Versions after 42 don't need to be added to this list,