Fix leaking Gtkosxapplication objects.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23191 57a11ea4-9604-0410-9ed3-97b8803252fd
Bug 632362 - Unable to create "reversing transaction" again after it is removed

From leaking references in g_object_get because both g_object_get
and g_value_set_object ref the object, which is only unreffed once.
Using g_value_take_object() instead prevents the double-ref.

BP

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23192 57a11ea4-9604-0410-9ed3-97b8803252fd