build: Use python postinstall script

This is what most GNOME modules now use instead of a shell script,
which makes sense given that the build system itself is written
in python.

This particular copy comes from nautilus ...

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/920

meson/postinstall.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+
+import os
+import subprocess
+
+prefix = os.environ.get('MESON_INSTALL_PREFIX', '/usr/local')
+datadir = os.path.join(prefix, 'share')
+
+# Packaging tools define DESTDIR and this isn't needed for them
+if 'DESTDIR' not in os.environ:
+    print('Updating desktop database...')
+    desktop_database_dir = os.path.join(datadir, 'applications')
+    if not os.path.exists(desktop_database_dir):
+        os.makedirs(desktop_database_dir)
+    subprocess.call(['update-desktop-database', '-q', desktop_database_dir])
+
+    print('Compiling GSettings schemas...')
+    schemas_dir = os.path.join(datadir, 'glib-2.0', 'schemas')
+    if not os.path.exists(schemas_dir):
+        os.makedirs(schemas_dir)
+    subprocess.call(['glib-compile-schemas', schemas_dir])