Add missing software to dependencies

Adds screen and trayer.
Add gksu to dependency list

Required by xmonad config for reboot/shutdown shortcuts.
Prevent nautilus from drawing the desktop

This can cause a problem with windows (particularly, xmobar) being drawn
beneath the desktop window. We don't want/use a desktop, so let's
prevent nautilus from ever drawing one.
Add slack.com support to Pentadactyl

Also, fix some overridden keyboard shortcuts for github.
Properly scale the wallpaper across large screens
Set up the trackpad better

Insteaad of a right-click hot-zone (which causes trouble moving then
clicking) let's use two fingered taps for a right click, and 3 for a
middle click (useful for 'Open in New Tab').
Fix trayer background coloring

Force the background color to the same as the xmobar window. This fixes
the weird issues I saw when using transparency.