Improvements to chrome.windows APIs and tests

note: there is some flakiness in these tests. If they seem to have stalled, just close any windows opened by the tests and refresh the test page.

Additionally: 
- test 'create incognito window' will not pass if the extension doesn't have access to incognito windows.
- test 'create panel' will not pass if the enable panels flag is not set (this is the default value for that flag in the devel and beta channels, but not the default value for it in the stable channel).