Given the way the autosave feature was awkwardly bolted on to the
composer, an EExtension seemed like a natural fit. And it helped
clean up some object lifecycle hacks (and bugs).
What we have now is a new module consisting of two EExtensions:
EComposerAutosave extends EMsgComposer and determines when to
kick off an asynchronous autosave operation.
EComposerRegistry extends EShell and offers to restore orphaned
autosave files on startup (which is also asynchronous now).
e-autosave-utils.c holds the actual asynchronous functions and a few
other miscellaneous utility functions.
Source code for the new module lives in /modules/composer-autosave.
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