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

po/POTFILES.in

@@ -1,5 +1,5 @@
 # List of source files containing translatable strings.
-data/gnome-calendar.desktop.in.in
+data/org.gnome.Calendar.desktop.in.in
 data/org.gnome.calendar.gschema.xml.in
 [type: gettext/glade]data/ui/edit-dialog.ui
 [type: gettext/glade]data/ui/menus.ui

po/POTFILES.skip

@@ -1,2 +1,2 @@
 # List of skipped of translation files
-data/gnome-calendar.desktop.in
+data/org.gnome.Calendar.desktop.in