Gnumeric 1.12.15

Andreas:
	* Fix chart size in ODF import. [#728278]
	* Improve chart roundtrip through ODF. [#728197]
	* Export centre-across-selection in a way LO can understand. [#726010]
	* Improve format roundtrip through ODF.

Morten:
	* For xlsx, don't complain over "ext" elements.
	* xlsx graph import improvements.
	* Attempt a fix for tabs appearance with gtk+ 3.12.  [#728465]
	* Make it possible to have all callables as python functions. [#728611]
	* Improve format roundtrip through ODF.
	* Plug leaks.
	* Make search dialog non-modal again.  [#728817]

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