Java installation cannot be cancelled once started

[machawk1]

Are you submitting a bug report or a feature request?

Feature request

What is the current behavior?

When misinterpreting the Java installation check (per #100), Java starts being installed in an un-cancelable process.

What is the expected behavior?

A Cancel/Stop button ought to be included in the UI.

What's your environment?

macOS 10.14, WAIL 1.2.0-beta3

Other information