Update glibmm to 2.20.0.

2.20.0 (stable):

* Documentation fixes (Daniel Elstner)

2.19.8 (unstable):
(2.19.4 to 2.19.8 were skipped)

* Added FILE_CREATE_REPLACE_DESTINATION enum value.
* Added Error::TOO_MANY_OPEN_FILES error code.
* Fix some compiler warnings about extra ;s.
  (Murray Cumming)

2.19.3 (unstable):

* New API:
  * BufferedInputStream: buffer_size property
  * BufferedOutputStream: buffer_size, auto_grow properties
  * DataInputStream: byte_order, newline_type properties
  * DataOutputStream: byte_order property
  * FilterInputStream: close_base_stream property, get/set_close_base_stream()
  * FilterOutputStream: close_base_stream property, get/set_close_base_stream()
  * UnixInputStream: fd and close_fd properties, get_fd(), get/set_close_fd()
  * UnixOutputStream: fd and close_fd properties, get_fd(), get/set_close_fd()
* Make licenses consistent with eachother (library gpl vs. lesser gpl, etc)
  (Deng Xiyue)
* Fixed some problems when building with exceptions disabled (Murray Cumming)
* Build fixes (Theppitak Karoonboonyanan)

2.19.2 (unstable):

* File: Added make_directory_with_parents() method overload with no parameter.
  Bug #566003 (Philip Belemezov).
* NoteTree: Do not use a non-ASCII dash character in a comment, to
  avoid the MSVC++ warning C4819 with Visual Studio 2008.
  (Tao Wang) Bug #568072
* spawn_*() functions: Added alternative API for when
  GLIBMM_EXCEPTIONS_ENABLED is not enabled. (Murray Cumming)
  Bug #565487 (Jonathon Jongsma)
* uri_*() functions: Fix memory leak.
  (Jonathan Jongsma) Bug #566845 (Jason Kasper)
* ustring: Add an overload which takes only a format string and no arguments
  to be substituted. (Daniel Elstner) Bug #506394.

2.19.1 (unstable):

* Fix accidental ABI breakage in 2.19.0 caused by adding default handlers for
  new Gio::Drive signals

2.19.0 (unstable):

* wrapped new glib 2.19.x API
  * Gio::AppInfo: get_commandline(), can_delete(), do_delete(),
    reset_type_associations(), launch_default_for_uri()
  * Gio::Drive: new signals changed, disconnected, and eject_button
  * Gio::Icon: create(std::string), to_string()(),
  * Gio::Mount: guess_content_type_sync(), is_shadowed(), shadow(), unshadow()
  * Gio::content_type_from_mine_type();
  * Gio::DesktopAppInfo: create_from_keyfile()
  * Gio::Emblem: new class
  * Gio::EmblemedIcon: new class
  * Gio::MemoryOutputStream: new class
* Bugs Fixed:
  * 555743 - warning in glibmm
  * 562716 - warning fixing cause more warnings.
  * 396963 - Add Glib::signal_idle().connect_once()
* Thanks to contributions from Przemys��aw Grzegorczyk, Dave Foster, Hubert
  Figuiere, and Jonathon Jongsma for changes in this release

(jmcneill)