Visually display when operations are not-applicable

[alukach]

In the spirit of "data-first" GIS, menu items should visually display whether they are applicable to the current selection.

This would likely be done by firing a signal on any change to the L.Dropchop.app.layerlist.selection.list object. All operations would subscribe to this signal, and alter their dom element to have a lighter text color if not applicable. I think a user should still be able to click on the operation and attempt to use it, as the error message returned from failed validation provides feedback to the user on the reason why an operation is not applicable.

[mapsam]

I always assumed the operation wouldn't be clickable, but we would show a tooltip when the user hovered over a disabled button, but this is also interesting - and seems like a good way to get the user to register that their data do not match the operation.

[alukach]

A tooltip showing why an operation is disabled would also work. I just want to make sure that there's some feedback available as to why it's disabled.

[mapsam]

Make operations available when data selection matches requirements for the operations: