Feature: implement mouse bindings

Feature: implement mouse bindings

A configured mouse binding (for example `bindsym button3 kill`) runs
its command when the mouse button is pressed over parts of a container.

If the binding has no modifer, it will only run when the button is
clicked on the window titlebar.

Otherwise if the binding has a modifier, it will run over the titlebar
or any part of the contained window.

fixes #558