build: Replace Autotools with Meson

Quick rundown of benefits:

- Much faster:
  - Autotools (with autogen): 22 seconds
  - Meson: 7 seconds
  - Meson (with ccache): 2 seconds

- Simpler:
  - ~1000 lines smaller
  - Single simple language

- Potentially better Windows (Visual Studio) support

What is not done:

- Complete Windows support
- OSX support (easy)

Closes #2013
Closes #1937
Closes #1803

meson_post_install.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+
+import os
+import sys
+import subprocess
+
+prefix = os.environ.get('MESON_INSTALL_PREFIX', '/usr/local')
+datadir = os.path.join(prefix, 'share')
+with_thememan = sys.argv[1] == 'true'
+
+# Packaging tools define DESTDIR and this isn't needed for them
+if 'DESTDIR' not in os.environ:
+    print('Updating icon cache...')
+    subprocess.call(['gtk-update-icon-cache', '-qtf',
+                     os.path.join(datadir, 'icons', 'hicolor')])
+
+    print('Updating desktop database...')
+    subprocess.call(['update-desktop-database', '-q',
+                     os.path.join(datadir, 'applications')])
+
+    if with_thememan:
+        print('Updating mime database...')
+        subprocess.call(['update-mime-database',
+                         os.path.join(datadir, 'mime')])