#264 Allow mapping keys to mouse buttons

[Magnus Henoch]

While trying to play The Settlers under DOSBox, I discovered that this game requires something the manual calls a "special click": click the left mouse button while holding down the right mouse button. It turns out that it's impossible to perform this on a MacBook trackpad.

Thus I came up with this patch, which lets you map keys to mouse buttons. Now I can use a key on the keyboard as the right mouse button, and use the trackpad for the left mouse button.