Bookmarks: Check validity of insert position
Bookmarks: Check validity of insert position

Before the C++11 port the underlying container was Glib list and the
g_list_insert interprets position parameter as "If this is negative, or
is larger than the number of elements in the list, the new element is
added on to the end of the list.".

=> this commit revamps backward compatibility with pre C++11 libfm-qt.

lxde/libfm-qt,lxde/libfm-qt,vc-01/libfm-qt,vc-01/libfm-qt