Rebase scratchpad mappings onto Tab

Far too often I miss the grave and hit Escape; Tab is safer and also
more intuitive, should one remember the ancient windoze times...

dotfiles/i3/config

 bindsym $mod+Shift+Page_Up   move container to workspace prev_on_output
 bindsym $mod+Shift+Page_Down move container to workspace next_on_output
 
-bindsym $mod+Shift+grave move scratchpad
-bindsym $mod+grave scratchpad show
+bindsym $mod+Shift+Tab move scratchpad
+bindsym $mod+Tab scratchpad show
 
 # mark mode
 bindsym $mod+equal               mode "mark"