Allow adding more than 1 blank keyframe

Allows to add blank animation layers as the selection.

I talked briefly with emmet about this and it could be a good addition for some workflow cases. However it needs a bit of UI testing, so before fixing the issues I need to test a bit if the change is indeed feels ok with the workflow design we have for animations.

As it is now I think it needs polish, i.e. If the current layer is locked and the selection goes beyond more than one layer no blank layer is added. if current layer is no locked it will ad blank frames on non locked frames. Which is inconsistent.

BUG:459455

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