Rewrote translateEntities in an imperative way.

Recursive version could cause stack overflow errors when dealing with
very long texts.
Added description in feedInfo and gui properties saving

- FeedInfo class now has a description field. This field contains the
first 500 chars of the feed description/summary tag.
- The 500 char limit is performed with a specialization of
std.range.take() for utf-8 strings that make sure it does split between
2 bytes of the same code point. Otherwise invalid utf would be produced.

- Now elements of the GUI such as window size and column widths are save
to file and restored between each run.