Log Message:
vala: update to 0.38.4.

Vala 0.38.4
===========
 * Various bug fixes:
  - Enforce name-length >= 3 for structs using GType [#764041]
  - Report error for missing type-parameter on enclosing type [#587905]
  - Discover invalid member access to instance field [#790903]
  - Improve error output of mismatching overriding methods
  - vala: Don't transform an explicit "null" into valid format-string \ 
[#791215]
  - codegen: Don't create null-safe destroy-wrapper for GenericType [#791283]
  - Improve error message for missing type-parameter on enclosing type [#587905]

 * GIR parser:
  - Allow change of parameter names
  - Handle metadata for fields inside a transparent union
  - Accept setters with boolean return-type as valid property-accessor
  - Don't mark simple-type out-parameters with '?' as nullable

 * Valadoc
  - Acknowledge possible properties in structs [#784705]
  - Fix fatal typo in GtkdocRenderer.visit_symbol_link() [#790266]
  - libvaladoc: Actually resolve "percnt;" to '%'

 * Bindings:
  - gobject-2.0: Add some missing symbols
  - gstreamer-1.0: Bind GST_DEBUG_BIN_TO_DOT_FILE/*_WITH_TS [#785215]
  - gstreamer-1.0: Update from 1.13+ git master
  - gtk+-3.0: Update to 3.22.26+9ce824d3
  - webkit2gtk-4.0: Update to 2.19.2
  - glib-2.0: Fix potential null pointer dereference in string.joinv() [#791257]
  - gobject-2.0: Add missing GType constants for fundamentals [#791023]