Now a 'More' item is added to the pie when there are too many items. It opens a sub-pie that contains the remaining items.
Note that this only touches operator-enum pies (like the object mode pie is), it is not a complete support for pies with more than 8 items. For this further design and code work would be needed, but this is too urgent to wait for that.
This is a better fix for T46973, should definitely be applied for 2.77 release.
Patch D1800 by myself with some edits by @campbellbarton, thanks!
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