UI: Improve the way splitting UI

[tordans]

Right now, the scissors are hidden when the way cannot be split (because it is part of a relation).

Ideally, the UI would explain this situation to the user. Something like a grayed out scissor with a popover explaining the issue.

Maybe even hinting at a workaround: Save, open in iD, Split, Save in iD, Reload ParkingLanes, Continue editing.

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Related: https://github.com/zlant/parking-lanes/issues/38

[tordans]

I plan to work on this a some point.