Allow to turn the launcher upside-down

When the BFB was sent to the launcher in Oneiric, users lost, among other funcionality (see bug #825146), the top-left hot corner which facilitated to evoke both the dash and the launcher. With the BFB on the upper side of the launcher, despite the advantages of having it on the launcher, it is now harder to reach for because it's not in a corner anymore and it's very close to window buttons and other important applications buttons (e.g. Firefox's back button), so the user is subject to constant accidental opening of the dash or accidental pushing of application/window buttons.

Moving the BFB to the bottom of the launcher would restore it back to a corner, making it comply with Fitt's law again, and would send it away from applications buttons. This could also improve the discovery of the dash button by new users, since the bottom-left corner of the screen is where the "start button" is found in KDE, LXCE and Windows.

However, simply moving the dash button to the bottom-left would cause it to be isolated from the applications launchers and consequently increase mouse travel, so this would be better implemented if the users had the option of turning the launcher position upside-down, i.e. the icons aligned to the bottom and the concertina going towards the top of the screen.

In addition, getting a hot corner back would allow users of applications with sidebars (like Inkscape) who are annoyed by the launcher constantly showing up accidentally to remove the reveal mode from the left without huge mouse-travel penalties.