Floating compose panel

[passcod]

From #255:

Instead of having the compose panel push the columns to the right, it appears "on top" of the rest of the view, so the display doesn't move. It also makes it faster (less animation / rendering!). I added some shadows to make it clear that it's above.

This was my implementation, for reference:

    .app-content.is-open .drawer[data-drawer=compose] {
        box-shadow: 3px 3px 3px 0px rgba(0, 0, 0, 0.2);
        left: 0;
        z-index: 999;
    }

    .app-content.is-open {
        transform: none !important;
        margin-right: 0 !important;
    }

[eramdam]

I consider Better TweetDeck to be in a feature freeze state due to the impending release of the newer TweetDeck and will instead shift my focus to building an alternative client. As such, I'm marking this issue as closed for the time being.
See the following issue for details #848