Create an interactive porting tool

`blueprint-compiler port` interactively ports a project to blueprint.
It will create the subproject wrap file, add it to .gitignore, decompile
your GtkBuilder XML files, emit code to copy and paste into your
meson.build file, and update POTFILES.in.

It can't quite handle all of the features the forward compiler can, so
it will skip those files.

docs/setup.rst

@@ -2,8 +2,24 @@
 Setup
 =====
 
-Use Blueprint in your project
------------------------------
+Setting up Blueprint on a new or existing project
+-------------------------------------------------
+
+Using the porting tool
+~~~~~~~~~~~~~~~~~~~~~~
+
+Clone [blueprint-compiler](https://gitlab.gnome.org/jwestman/blueprint-compiler)
+from source. You can install it using `meson _build` and `ninja -C _build install`,
+or you can leave it uninstalled.
+
+In your project's directory, run `blueprint-compiler port` (or `<path to blueprint-compiler.py> port`)
+to start the porting process. It will walk you through the steps outlined below.
+It should work for most projects, but if something goes wrong you may need to
+follow the manual steps instead.
+
+
+Manually
+~~~~~~~~
 
 blueprint-compiler works as a meson subproject.
 
@@ -26,6 +42,8 @@ blueprint-compiler works as a meson subproject.
 
       /subprojects/blueprint-compiler
 
+#. Rewrite your .ui XML files in blueprint format.
+
 #. Add this to the ``meson.build`` file where you build your GResources:
 
    .. code-block:: meson.build
@@ -44,10 +62,3 @@ blueprint-compiler works as a meson subproject.
 
       dependencies: blueprints,
 
-#. Convert your .ui XML files to blueprint format. In the future, an automatic
-   porting tool is planned.
-
-
-.. warning::
-   The blueprint compiler flattens the directory structure of the resulting XML
-   files. You may need to update your ``.gresource.xml`` file to match.