add toggle focus between floating and tiled layers

Adds focus_floats(), toggle_focus_float_tiled(), and their IPC
bindings so users can bind a key to swap focus between the floating
and tiled layers.

toml-config/src/config.rs

@@ -83,6 +83,7 @@ pub enum SimpleCommand {
     FocusHistory(Timeline),
     FocusLayerRel(LayerDirection),
     FocusTiles,
+    ToggleFocusFloatTiled,
     CreateMark,
     JumpToMark,
     PopMode(bool),