JVET-N0400: Signal max number of triangle merge candidates

Merged Xiang Li requested to merge XiangLi/VVCSoftware_VTM:jvet-n0400-trianglenum into master

JVET-N0400, JVET-N0447, JVET-N0500, JVET-N0851: Signal max number of triangle merge candidates, align triangle merge candidate number and regular merge candidate number

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