Issue 24224

I happened by chance to try some bitmap fill for part of graphs created with the
spreadsheet module. However, I did not like it and reverted to single color area
fills. However, when saving the document after this, its size was significantly
increased. By unzipping and examining the xml file I discovered the problem: the
folder for the above graph still contained a copy of all the bitmap fill I tried
but did not use in the end! Even worse, those bitmaps were replicated in the xml
a few times since I copied and pasted the graph in various parts of the document
after trying - but rejecting! - those bitmap fills. In all I had some 1.1 MB
added to the file made by 8 replicas of the 3 bitmap files tried for the fills.
And this  without ever using any of these bitmaps for the actual document!
To summarize, the xml should not contain:
- copies of bitmaps not actually used in the document; when any such bitmap
previously used is dismissed it should be deleted from the xml file at the next
"save" operation;
- replicate copies of identical bitmap files used in various parts of the
document; for example, if I make a presentation file cointaining some 20 graphs
from the spreadsheet app, and all of them have the same 80KB background bitmap
image, saving this same bitmap separately for each graph means adding some 1.6
MB to the document size - the bitmap should be inserted in the xml file only
once as long as it is exactly the same in different instances.