bnc#883684: Make chart objects exempt from automatic unloading.

The Memory option page contains "Number of objects" setting which controls
how many OLE objects can be loaded at any given time.  When the number of
total OLE objects exceeds this number the oldest ones get unloaded.  Note
that the total number is across all open documents, not per document.

Technically, chart objects are OLE objects, but unlike other OLE objects
that are embedded Calc, Writer, Draw documents, chart objects normally
"belong" to the host document.  It therefore makes no sense to subject
chart objects to this automatic unloading.

In the future we may want to apply this exemption to other types of OLE
objects, like formula objects for example..  Or maybe this setting can be
removed altogether...

Change-Id: I7dd92550880efb4db8fc843cf1c915c712166da4