modes for keybindings

[sunaku]

Keybinding modes are basically a different set of keybindings for different tasks (e.g. no interference; client growing; client positioning; and other uses). They should have a common escape sequence (like Control-Alt-Esc) which returns you to the normal keybindings.

Also add a status bar applet that shows the current keybinding mode. You should be able to right-click it to see a menu of all modes (and switch to the clicked one).