Enable/disable layers, segments and more in the model tree

[stfnp]

When experimenting with different variants, it could sometimes be useful to disable selected features of the current bow model, like layers and profile segments. Maybe a menu item "Enable" or "Disable" could be added to the respective model tree items. Disabled items are then greyed out and ignored in the simulation.

Maybe start with only layers, since this is the easiest and most obvious case.