Remember recently-used charsets

[eyalroz]

Leafpad allows us several options for the charset when opening a file, and an "Other codeset..." if what we need isn't on the list. This is ok, but - if you type in a charset, it isn't added to the list. It should, including the next time you open Leafpad. Removing items from the list could happen on a least-recently-used basis (with some items never removed, like UTF-8 and Latin-1). Alternatively, we should be able to list some charsets in some kind of file which leafpad reads (I don't even know whether it has a configuration file or not).