Simple-tab-groups: STG icon spins constantly in private window

[ryetek]

Describe the bug
Whilst I appreciate that STG does not support private (incognito) mode from version 3.2 the STG icon constantly spins when in a private window.

To Reproduce
Steps to reproduce the behavior:

1. Press CTRL+SHIFT+P to start a private window
2. STG icon will constantly spin

Expected behavior
STG icon should be "greyed out" and become unavailable. Spinning animation should stop.

Desktop (please complete the following information):

• OS: Windows 8.1Pro
• Version: 3.2
• Firefox Version: 62.0

[Drive4ik]

Hi! Thanks for find bug, this fix will available on next release of STG