Bug 1022755 - do not scroll if we're not in the right panel r=azasypkin

* do not try scroll if we're not in the "thread" panel
* Navigation.currentPanel should be null when we're sliding
* Navigation.slide() now correctly removes the event listener
Merge pull request #21280 from julienw/1022755-dont-scroll-if-in-wrong-panel

Bug 1022755 - do not scroll if we're not in the right panel r=azasypkin