attach panel to ToggleButton in FF29

Uses hack from kind folks over in #jetpack. This allows us to stop using
toolbarwidget in 29, which has been the cause of numerous reported issues.
revert to built-in sdk/widget for < 29

toolbar widget gives users a consistent experience across FF < 29 and >= 29.
After releasing the alpha, it became clear that this was not worthwhile. Users
of < 29 expect addons to appear in the addon bar (and they can always be
dragged to the toolbar, which works well). Additionally, using toolbarwidget
created a problem where moving the icon from the toolbar to the addon bar did
not lead to the icon being properly resized.
fix broken dependency resolution in test
Remove documentation references to toolbarwidget
Add instapaper to cookieblocklist.txt
Make third-party cookie pref warning clearer
Remove weird white border around icons
Update changelog and version bump