Add default support for scrolling with the mouse wheel.

[ccrusius]

Also changes the scrollback() function so that the amount to scroll can be configured as a fraction of the screen height.

This fixes #103.

[gwn]

Hi, I'm a long time dvtm user. Just wanted to note that this patch seems to be working properly and I'm very happy with the feature, although I don't know about the implementation details. Thank you @ccrusius !

[kugurerdem]

I have also tried this patch and it seems to work very well. Thanks a lot @ccrusius !

[rpmohn]

Shameless plug for a4 https://a4term.com which is a rewrite of dvtm and implements configurable scrolling with the mouse wheel by default. You can specify a number of lines to scroll (with an integer) or a percentage of a screen (with a decimal number). You can scroll whichever window the mouse is hovering over, not just the selected window. You can also use the mouse for additional configurable actions by clicking on a tag in the status bar, the layout symbol, and more. End of shameless plug.