qga: Protect "owner-change" event loop by checking current owner
ui/gtk: Change callback parameter type to GdkEventOwnerChange
qga: Rename guest-session-set-clipboard to session-set-clipboard
qga: Remove the old clipboard owner changed callback code
qga: Fix the place of g_free()
ui: Add a shared gtk-clipboard.h
qga: Add stub for command guest-session-set-contents-with-target