Two major features for v3

[fwdekker]

Implements and tests two major features:

• Variable settings-only dialog based on user's templates
• Resettable changes to symbol sets and dictionaries

The latter required the introduction of the SettingsState class, which references the upper-level Settings objects so that schemes can use those. This also makes the only remaining major feature easier to implement, as schemes now have the ability to refer to each other using the SettingsState.

There's still a large amount of important, non-major features remaining after that, but they will likely not require major design overhauls.

[fwdekker]

Apparently I'm out of credits on Travis so I can't run builds anymore...