Rename e-fsutils to e-file-utils.  This is where we'll add asynchronous
functions for common file I/O operations with EActivity integration.

Start with e_file_replace_contents_async() (and corresponding finish()
function).  This is a simple wrapper for g_file_replace_contents_async()
which also returns an EActivity.  It replaces e_write_file_uri().

Also redesign EIOActivity to -contain- a GAsyncResult rather than
implement the interface for itself.  This is easier for now but I may
change my mind again when I figure out how to tie centralized error
reporting into the EActivity framework.