project: Reorganize files

Add a build-aux/ folder and move the Flatpak and
Meson auxiliary files there.

build-aux/meson/meson_post_install.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+
+import os
+import subprocess
+
+install_prefix = os.environ['MESON_INSTALL_PREFIX']
+icondir = os.path.join(install_prefix, 'share', 'icons', 'hicolor')
+schemadir = os.path.join(install_prefix, 'share', 'glib-2.0', 'schemas')
+
+if not os.environ.get('DESTDIR'):
+  print('Update icon cache...')
+  subprocess.call(['gtk-update-icon-cache', '-f', '-t', icondir])
+
+  print('Compiling gsettings schemas...')
+  subprocess.call(['glib-compile-schemas', schemadir])