app: make DBus-activatable

This commit installs a service file, renames the desktop file
to follow the bus name, and adds the DBusActivatable key to the
desktop file, following
https://wiki.gnome.org/HowDoI/DBusApplicationLaunching

Updates POTFILE files accordingly

data/org.gnome.Calendar.desktop.in.in

@@ -0,0 +1,12 @@
+[Desktop Entry]
+_Name=Calendar
+_Comment=Access, and manage calendar
+Exec=gnome-calendar
+Icon=gnome-calendar
+Terminal=false
+Type=Application
+StartupNotify=true
+OnlyShowIn=GNOME;
+Categories=GNOME;GTK;Utility;Core;
+MimeType=text/calendar;
+DBusActivatable=true