build: Switch to Meson’s gnome.post_install function

Instead of using a custom post install script.
This drops the GTK 3 dependency.

Required bumping minimum Meson version to 0.59
for the `update_desktop_database` kwarg.

https://mesonbuild.com/Gnome-module.html#gnomepost_install

build-aux/meson_post_install.py

@@ -1,18 +0,0 @@
-#!/usr/bin/env python3
-# -*- coding: utf-8 -*-
-
-import os
-import subprocess
-
-install_prefix = os.environ['MESON_INSTALL_PREFIX']
-
-if not os.environ.get('DESTDIR'):
-    icon_cache_dir = os.path.join(install_prefix, 'share', 'icons', 'hicolor')
-    if os.path.exists(icon_cache_dir):
-        print('Updating icon cache…')
-        subprocess.call(['gtk-update-icon-cache', '-qtf', icon_cache_dir])
-
-    desktop_database_dir = os.path.join(install_prefix, 'share', 'applications')
-    if os.path.exists(desktop_database_dir):
-        print('Updating desktop database…')
-        subprocess.call(['update-desktop-database', '-q', desktop_database_dir])

meson.build

@@ -146,8 +146,11 @@ endif
 subdir('malcontent-client')
 if get_option('ui').enabled()
   subdir('malcontent-control')
+
+  gnome.post_install(
+    gtk_update_icon_cache: true,
+    update_desktop_database: true,
+  )
 endif
 subdir('pam')
 subdir('po')
-
-meson.add_install_script('build-aux/meson_post_install.py')