update for DESKTOP_ENTRIES framework

[Max Brazhnikov]

Hi folks,

I propose an update for DESKTOP_ENTRIES:
http://people.freebsd.org/~makc/patches/desktop_entry_20130102.diff

Changes:
- update list of categories to the version 1.1 of Desktop Menu Specification
- require Icon field to follow Desktop Entry Specification (the field must be
  absolute path or icon name if icon theme is used)
- require main category to be set (listed in DESKTOP_CATEGORIES_MAIN)
- remove deprecated Encoding key
- allow StartupNotify field to be empty string
- be less strict when generating filename for desktop entry (to improve
  readability when full path to executable is used)
- use port ${COMMENT} if comment field is not set
- add GenericName key (comment field will be used)

About 20 ports will throw warnings about icon field, several ports don't set
main category, about 60 use non-standard 'Application' for categories.
I'm willing to fix them all.

Review and suggestion are welcome!

Happy New Year!
Max