Translate |textdomain| without |include|

Before this commit, the |textdomain| statement was translated as:

  include I18n
  textdomain "foo"

This caused problems when the |textdomain| statement was used inside a
function -- see #10.

This commit changes the translation to:

  textdomain "foo"

The |include| is not needed anymore as all clients & modules include
I18n by default (because it is included in their base class).

Fixes #10.
Do not treat |textdomain| statements specially

Until now, the |textdomain| statements at module/client toplevel were
treated specially and placed into generated class toplevel, not into the
|main| or |initialize| method as other statements. This was necessary
because the generated code contained an |include| statement. This is not
true anymore (since the last commit), so let's treat the |textdomain|
statements like any other ones.