The 'movable' style attribute of cells had the opposite effect as what
would be expected. When set to false, cells could be moved, and when true,
cells couldn't be moved.
This is something that got lost in translation in the conversion from mxGraph.
Previously, it assumed that if 'movable' had any value, it must be the string "0". This
attribute got changed to a boolean in Typescript, but it looks like the
check never got updated leading to this strange behaviour.
The fix is fairly simple.
If 'movable' is undefined, then we assume that it's true.
If not, then we check it like a regular boolean.
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