Deprecate recently_edited option

[yani.iliev]

recently_edited option is being set when a plugin or a theme file is edited by the builtin Editor.
A search, in the WordPress 3.8 source code, tells me that the option is never being read.
I suggest that we deprecate that option and replace it by an action where plugins can hook and get the value of the edited file.

[nacin]

I think keeping this is potentially useful information we may wish to use at a later point, and something that core probably could use, too.

[maximinime]

At least fix the option so that the paths it contains are relative to the install folder rather than absolute for the local file system.

[MikeHansenMe]

@maximinime can you comment on the benefits there will be if this is changed from absolute? I was trying to figure out the pros/cons and I am thinking the main time this would change is if you switch hosting providers. In that case I was thinking it may be a good thing to know which file was changed(before or after).

[maximinime]

I'm not a fan of storing absolute paths when it's not required. Given that this option always contains a file that is in or below the web root, it makes sense to refer to the same file when migrating servers, rather than having a potentially invalid path.