Fix #1184 -- Replace loading gif with css animation

[codingjoe]

You can test it by loading a page, then disabling the network in the browsers debug console followed by opening any toolbar tab.

The whole thing looks like this:

[jdufresne]

Nice! This is really neat. Thanks for contributing it.

[codingjoe]

@jdufresne I removed the display property as suggested. I guess you are most familiar with the merge and release proceedings. I'd leave that to you, if that's ok.

[matthiask]

Thank you, that's awesome!