Indicate when the extension is on/off

[sambuddhabasu]

Presently, we can stop the extension by clicking on Stop Injecting. However, to denote the running state of the extension, there is no on/off visual.
In Safari extensions, we cannot have a modal button so the best thing to do in my opinion, is to have 2 different images, one with the on and one with the off text. Once the Stop Injecting is pressed, the image with the off text will be shown as the image of the toolbar item. On clicking Start Injecting, the image of the toolbar will be change again.
@smcgregor What do you think about this approach?

[smcgregor]

Sounds good.