Current src/gnome/gnucash.desktop.* files generates warnings with desktop-file-validate, showing that they use deprecated or not standard values.

### Need to be fixed :
gnucash.desktop.in: error: value "application/x-gnucash" for string list key "MimeType" in group "Desktop Entry" does not have a semicolon (';') as trailing character
gnucash.desktop.in: warning: key "Encoding" in group "Desktop Entry" is deprecated
gnucash.desktop.in: warning: value "Application;Office;Finance;" for key "Categories" in group "Desktop Entry" contains a deprecated value "Application"

### Should not be fixed (is modified during make) :
gnucash.desktop.in: error: file contains key "_Name" in group "Desktop Entry", but key names must contain only the characters A-Za-z0-9- (they may have a "[LOCALE]" postfix)
gnucash.desktop.in: error: file contains key "_Comment" in group "Desktop Entry", but key names must contain only the characters A-Za-z0-9- (they may have a "[LOCALE]" postfix)
gnucash.desktop.in: error: required key "Name" in group "Desktop Entry" is not present
gnucash.desktop.in: error: filename does not have a .desktop extension