add courtesy keysig to last measure even if it is followed by frame

[sammik]

Resolves: #22256
Resolves: #19759

• creating courtesy keysig was disabled by MeasureLayout::createEndBarLines(measure, **false**, ctx);, which sets ElementFlag::KEYSIG of measurebase to false, so before adding trailer, we need to re-createEndBarLines with truthy isLastMeasureInSystem value
• this was already (almost) done, with one exception - it processed last measureBase of system, but we need last real measure (not frame)

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