Hide global environment variables by default

[tlmii]

1. Adds a flag to the environment variables viewmodel specifying whether it is a spec variable or a global one. Values are always taken from the effective list regardless.
2. By default, environment variables dialog shows only the ones from spec.
3. Adds a toggle at the top of the dialog to switch between spec-only and all.
4. Hides the toggle on containers, where there's (currently) no difference

Resolves #167

[tlmii]

Screengrab:

A couple things we need to settle on here:

1. What icons should we use? I picked two to get us started, but they're not ideal.
2. What should the tooltips on the button say for the two different states? Currently it is "Show All" and "Show Spec Only".

cc @leslierichardson95 on these questions in particular

[davidfowl]

Does it load faster 😄