Add mouse support

[lproven]

It would be useful to be able to control the menus, and select text, using the mouse.

Both in X and on the console.

For comparison, the Tilde editor does this on the console via the gpm mouse driver.

[davidgiven]

I agree this is desireable, but it's both complicated (so many edge cases...) and is incompatible with the current fixed cursor line behaviour (see #89). So, I'd need to change that first. I actually think that's a good idea anyway.