Gnumeric 1.12.50

Morten:
	* Fix undo problem with sorting and cell comments.
	* Python loader cleanups.
	* Use Python's bool type instead of rolling our own.
	* Export file:// links to html.  [#569]
	* Export file:// links to xlsx.  [#569]
	* Improve handling of invalid formulae from xlsx.  [#574]
	* Introspection fixes.
	* Improve CELL("format",...)  [#576]
	* wk4 import improvements.  (For no good reason.)
	* Fix some -fsanitize=undefined problems.
	* Improve excel-specific xlsx import of validation.

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