default config: add mouse bindings

Long description:

By default, i3 is configured to close a window when the titlebar is
middle clicked (which is well-known behavior from web browsers)

Right click is configured to toggle floating / tiling (this would
normally open a context menu, but i3 will never have these).

To apply this patch, use:
curl http://cr.i3wm.org/patch/594/raw.patch | git am

@@ -37,6 +37,9 @@ bindsym Mod1+Return exec i3-sensible-terminal

 # kill focused window

 bindsym Mod1+Shift+q kill

+# kill focused window when the titlebar is middle clicked

+bindsym button2 kill

+

 # start dmenu (a program launcher)

 bindsym Mod1+d exec dmenu_run

 # There also is the (new) i3-dmenu-desktop which only displays applications

@@ -85,6 +88,9 @@ bindsym Mod1+e layout toggle split

 # toggle tiling / floating

 bindsym Mod1+Shift+space floating toggle

+# toggle tiling / floating when the titlebar is right clicked

+bindsym button3 floating toggle

+

 # change focus between tiling / floating windows

 bindsym Mod1+space focus mode_toggle