Upgrade to FEt 5.23.3

- Updated to Qt 5.3.2.
- Added a fet.desktop file (by Alexey Loginov).
- Fixed some comments concerning the GNU GPL license (reported by Alexey
  Loginov).
- Added a "make install" option (suggested by Alexey Loginov and other
    users, done by Zsolt Udvari and Liviu Lalescu).
- Added an icon (suggested by Alexey Loginov and other users).
- Added an option at compilation time so that FET automatically detects
  the system locale (suggested by Alexey Loginov and other users).