Add translator comments to the .desktop file

So hopefully we get fewer invalid translations that possibly break build.

data/org.gnome.Calendar.desktop.in

@@ -2,11 +2,13 @@
 Name=Calendar
 Comment=Access and manage your calendars
 Exec=gnome-calendar
+# Translators: Do NOT translate or transliterate this text (this is an icon file name)!
 Icon=org.gnome.Calendar
 Terminal=false
 Type=Application
 StartupNotify=true
 Categories=GNOME;GTK;Office;Calendar;Core;
+# Translators: Search terms to find this application. Do NOT translate or localize the semicolons! The list MUST also end with a semicolon!
 Keywords=Calendar;Event;Reminder;
 MimeType=text/calendar;
 DBusActivatable=true