[IMP] extension icon illegible in toolbar

[gjotten]

Using a white-background full logo makes it very unclear:

Use below square logo so that the timer is less branded (T can stand for "Timer") but still recognizably our work. Pad it out so it becomes more distinct on toolbar.

Then use:

• lightgrey background with white logo as icon for inactive timer
• Therp blue background, white logo on active timer (so color-inverted) for clarity