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

data/icons/Makefile.am

@@ -1,15 +0,0 @@
-icon_DATA = hexchat.png
-icondir = $(datadir)/icons/hicolor/48x48/apps
-
-hicolor_DATA = hexchat.svg
-hicolordir = $(datadir)/icons/hicolor/scalable/apps
-
-UPDATE_ICON_CACHE = gtk-update-icon-cache -f -t $(datadir)/icons/hicolor || :
-
-install-data-hook:
-	$(UPDATE_ICON_CACHE);
-
-uninstall-hook:
-	$(UPDATE_ICON_CACHE);
-
-EXTRA_DIST = $(icon_DATA) $(hicolor_DATA)

data/icons/meson.build

@@ -0,0 +1,9 @@
+icondir = join_paths(get_option('datadir'), 'icons/hicolor')
+install_data(
+  'hexchat.png',
+  install_dir: join_paths(icondir, '48x48/apps')
+)
+install_data(
+  'hexchat.svg',
+  install_dir: join_paths(icondir, 'scalable/apps')
+)