Bug 946987 - Add 2x tab images for Windows and use them for 1.25dppx and higher. r=mconley

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extra : rebase_source : 918ece60e8ba56c81fdfc7be4bbdd80f39dcf82f
Bug 977572 - catch drag end and drop events from bookmarks drag handler so we can clean up, r=mak

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extra : rebase_source : 6a725c15b2802d7bf641e22f9ae322974f6639cc
Bug 993344 - The "Show tab previews in the Windows taskbar" option doesn't appear in Tabs section for in-content preferences. r=mak77

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extra : rebase_source : 3dd669c50db388a0fc56bf3c91c9845208aa03ef