Make option to remove layer (entries) from legend

[yileifeng]

Implement functionality to delete layer entries that were user-added.

[RyanCoulsonCA]

To add on to this, we need to implement the 'More Options' menu in general. Even though we can't implement most of the features in the menu at the moment, it'd be good to have the menu in place to add the features in the future (remove layer being one of them).