CONFIG BREAK: kick the 'next'/'prev' commands, introduce 'focus left' etc.

Instead of the cryptic next/prev commands, we now provide an argument to the
focus command. Userguide and default config is updated.

i3.config

@@ -57,16 +57,16 @@ bindsym Mod1+Shift+j reload
 bindsym Mod1+Shift+l exit
 
 # Focus (Mod1+n/r/t/d)
-bindsym Mod1+n prev h
-bindsym Mod1+r next v
-bindsym Mod1+t prev v
-bindsym Mod1+d next h
+bindsym Mod1+n focus left
+bindsym Mod1+r focus down
+bindsym Mod1+t focus up
+bindsym Mod1+d focus right
 
 # alternatively, you can use the cursor keys:
-bindsym Mod1+Left prev h
-bindsym Mod1+Right next h
-bindsym Mod1+Down next v
-bindsym Mod1+Up prev v
+bindsym Mod1+Left focus left
+bindsym Mod1+Down focus down
+bindsym Mod1+Up focus up
+bindsym Mod1+Right focus right
 
 # Move
 bindsym Mod1+Shift+n move left