Add Loading indicator

[RangerMauve]

• Add a new attribute called loading to the omnibox which should be "true" or "false"
• Inside attributeChangedCallback detect the changing of loading and change the content of the submit button to a loading indicator or to the regular indicator based on the state.
• Send did-start-navigation and did-navigate to the UI from window.js
• Listen for did-start-navigation and did-navigate inside the ui scripts
• Change the loading flag in the omni box to true when starting navigation and false when finishing navigation.

Might also want to add a method to stop loading when people click on the loader?

Also, we should consider adding aria roles to mark it as active.