Associate Poedit with MO files.

Allow "opening" MO files too.  Poedit still won't edit them directly
(yet), but it will show an explanatory message to help clarify the
distinction between PO and MO files to users unfamiliar with gettext.
Different indicator of modified entries.

Don't use a star icon, it is confusing to use iconography that has an
established different meaning (favorite).

Instead, use a green line alongside the entries to indicate modified
content, similarly to how e.g.  Visual Studio does it.  A solid line
would be better, but harder to implement with wxListCtrl, so use an icon
that forms reasonably well looking dashed line.