Removing TabHighlighter + M-w for closing tab + small fix

 1. TabHighlighter was initially introduced to enhance the contrast of
    the tabbar while using multiple tabs. This went unused because of
    bugs and a general overhaul of the UI, and was dead code until now.
    I've deleted all the corresponding code for TabHighlighter.

 2. Pandock has already introduced a small number of keyboard shortcuts
    to make people's lives easier. This commit introduces another one,
    `M-w`, which closes the current tab.

 3. Flowdock presents an annoying message for FluidApp users on Mac,
    which is automatically cancelled by the code. Recently the FD guys
    changed the structure of the message, and a small fix was needed for
    this to keep working.