Updating devtools panel to show webview

[jalissia]

This PR adds code to the devtools panel that will show a webview pointed at the devtools.html.

• Implemented DevToolsPanel class to create the webview
• Added messaging proxy code that will transfer messages between the extension frame and the devtools frame
• Added more unit tests

Now when you select a target a blank webview will open up in a new tab. WebView content will come in future PR.

[xirzec]

Looks good; I would prefer fishing the values out of the mocks instead of assigning to parent scope variables directly.