* allow launcher-per-screen configuration * HUD and Dash follow mouse cursor * panel per-screen, also on non-top screens * spread per-screen * support the switch to a single launcher on primary monitor. Fixes: https://bugs.launchpad.net/bugs/780204, https://bugs.launchpad.net/bugs/844798, https://bugs.launchpad.net/bugs/844809, https://bugs.launchpad.net/bugs/947145, https://bugs.launchpad.net/bugs/954175. Approved by Michał Sawicz.