[gtkmm] (4 commits) Non-fast-forward update to branch wip/dboles/Button-Issue#58

[Daniel Boles <dboles src gnome org>]

The branch 'wip/dboles/Button-Issue#58' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  57c18a0... Button: Remove mentions of NULL args @ set_image()
  87b47b9... Button: Add unset_image()

Commits added to the branch:

  58df45d... Avoid unnecessary conversions between std::string and Glib: (*)
  679f3d2... Button: Remove mentions of NULL args @ set_image() (*)
  d269d17... Button: Explain how to unset image via prop proxy, (*)
  5c38770... Button: Add unset_image()

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