Log Message:
Update gdeskcal to 0.57.

Changes since 0.55:
- Full compatibility with GTK 2.4
- Uses EWMH window flags if gDesklets is installed
- Integrated patch for event locations by Priit Laes (thanks, Priit!)
- Fixed endless loop bug (thanks to Inigo Serna for locating the bug),
  aka Debian bug #221406
- Fixed slash and ampersand bugs in event summaries, aka Debian bugs #216064
  and #217697