[glibmm] (7 commits) ...Merge branch 'std-string-msvc-fixes' into 'master'

[Chun-wei Fan <fanchunwei src gnome org>]

Summary of changes:

  f9e197c... ustring.h: Avoid exporting classes with std::string members (*)
  2abf27f... variantdbusstring.h: Don't mark classes with GLIBMM_API (*)
  1c9772f... fileattributeinfo.hg: Avoid exporting classes with std::str (*)
  c4392f4... error.h Avoid exporting class deriving frm std::exception (*)
  29b9a10... Don't mark the Glib::Source class with GLIBMM_API (*)
  eabcc01... Clean up Visual Studio build files (*)
  19f08ff... Merge branch 'std-string-msvc-fixes' into 'master'

(*) This commit already existed in another branch; no separate mail sent