Reuse stack morph from line above in BPSwimlane

[JoeAtHPI]

Currently, each line in a BPSwimlane consists of a morph for each called method in the stack.
However, if a line above any other line already defined a stack morph for a method, which is also called in that order for current line, the height of the previous morph should increase to cover the current line instead of making a new morph.
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This will decrease the morph count noticeably.