Fix warnings and errors raised by desktop-file-validate command:
fidocadj.desktop: warning: value "Application;Graphics;" for key "Categories" in group "Desktop Entry" contains a deprecated value "Application"
fidocadj.desktop: warning: key "Encoding" in group "Desktop Entry" is deprecated
fidocadj.desktop: error: file contains key "MultipleArgs" in group "Desktop Entry", but keys extending the format should start with "X-"
Add 2DGraphics category (used for example by Gimp, feh and LibreOffice Draw)
fidocadj.desktop: warning: value "Application;Graphics;" for key "Categories" in group "Desktop Entry" contains a deprecated value "Application" fidocadj.desktop: warning: key "Encoding" in group "Desktop Entry" is deprecated fidocadj.desktop: error: file contains key "MultipleArgs" in group "Desktop Entry", but keys extending the format should start with "X-"