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/meson.build

@@ -0,0 +1,11 @@
+if get_option('with-plugin')
+  subdir('pkgconfig')
+endif
+
+if get_option('with-gtk')
+  subdir('icons')
+  subdir('misc')
+  subdir('man')
+elif get_option('with-theme-manager')
+  subdir('misc')
+endif