shew: Add small library for dealing with external windows

In order to support OpenExtensionPrefs()'s parentWindow parameter,
we will need the ability to make a window transient to an external
window identified by a string handle.

This takes the corresponding code from xdg-desktop-portal-gtk and
brings it into an introspectable form, likewise the counterpart in
libportal to export a string handle for a GtkWindow.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1087

meson.build

@@ -243,6 +243,16 @@ libgvc = subproject('gvc',
 )
 libgvc_gir = libgvc.get_variable('libgvc_gir')
 
+libshew = subproject('shew',
+  default_options: [
+    'package_name=@0@'.format(meson.project_name()),
+  ]
+)
+
+libshew_version = libshew.get_variable('package_version')
+assert(libshew_version == meson.project_version(),
+  'shew version does not match project version')
+
 if get_option('extensions_tool')
   extension_tool = subproject('extensions-tool',
     default_options: [