Implement selecting the next tiling/floating window (using "focus")

Also update documentation (manpage, userguide).
To make the code easier to read/write when checking if a client is
floating, introduce client_is_floating().

i3.config

@@ -18,9 +18,13 @@ bind Mod1+43 s
 # Default (Mod1+e)
 bind Mod1+26 d
 
-# Toggle tiling/floating of the current window
+# Toggle tiling/floating of the current window (Mod1+Shift+Space)
 bind Mod1+Shift+65 t
 
+# Go into the tiling layer / floating layer, depending on whether
+# the current window is tiling / floating (Mod1+t)
+bind Mod1+28 focus ft
+
 # Focus (Mod1+j/k/l/;)
 bind Mod1+44 h
 bind Mod1+45 j