Add a meson build system, correct icon size.

8min on this task (I'll semantic for all future commits).

data/meson.build

@@ -0,0 +1,8 @@
+install_data('icons/write-as.png',
+    install_dir: join_paths(get_option('datadir'), 'icons', 'hicolor', '128x128', 'apps')
+)
+
+install_data('write-as-gtk.desktop',
+    install_dir: join_paths(get_option('datadir'), 'applications'))
+install_data('write-as-gtk.appdata.xml',
+    install_dir: join_paths(get_option('datadir'), 'metainfo'))

meson.build

@@ -0,0 +1,3 @@
+project('write-as-gtk', ['vala', 'c'], meson_version: '>=0.40.1')
+
+subdir('data')